
f Wj V 1 0„< «/'/ • • "'iff 




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The Lunar "Crater" Copernicus 

Photographed from Xasmyth and Carpenter's plaster-of-paris model 
of the moon. 
In this model the topography of the moon is faithfully represented as 
seen with powerful telescopes. 



Astronomy in a 
Nutshell 

The Chief Facts and Principles 

Explained in Popular Language 

for the General Reader 

and for Schools 



By 

Garrett P. Serviss 



With 47 Illustrations 



G. P. Putnam's Sons 

New York and London 

Gbe "Knickerbocker press 

1912 



*$ 



Copyright, 1912 

BY 

GARRETT P. SERVISS 



Ube "terucfeerbocfcer press, 1*ew Jgotb 



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GC!.A«147 - /8 



PREFACE 

How many thousands of educated people, 
trained in the best schools, or even graduates 
of the great universities, have made the con- 
fession: " I never got a grip on astronomy in 
my student days. They did n't make it 
either plain or interesting to me ; and now I 
am sorry for it." 

The purpose of the writer of this book is to 
supply the need of such persons, either in 
school, or at home, after school-days are 
ended. He does not address himself to special 
students of the subject — although they, too, 
may find the book useful at the beginning — 
but to that vast, intelligent public for whom 
astronomy is, more or less, a " mystical mid- 
land," from which, occasionally, fascinating 
news comes to their ears. The ordinary 
text-book is too overladen with technical 
details, and too summary in its treatment of 
the general subject, to catch and hold the 
attention of those who have no special pre- 
liminary interest in astronomy. The aim 
here is to tell all that really needs to be told, 



iv Pref 



ace 



and no more, and to put it as perspicuously, 
compactly, and interestingly, as possible. 
For that reason the book is called a ' ' nutshell. ' ' 



The author has been sparing in the use of 
diagrams, because he believes that, in many 
cases, they have been over-pressed. There is 
a tendency to try to represent everything to 
the eye. This is well to a certain extent, but 
there is danger that by pursuing this method 
too far the power of mental comprehension will 
be weakened. After all, it is only by an 
intelligent use of the imagination that progress 
can be made in such a science as astronomy. 
The reader is urged to make a serious effort 
to understand what is said in the text, and to 
picture it in his mind's eye, before referring 
to the diagrams. After he has thus presented 
the subject to his imagination, he may refer 
to the illustrations, and correct with their aid 
any misapprehension. For this reason the 
cuts, with their descriptions, have been made 
independent of the regular text, although 
they are placed in their proper connections 
throughout the book. 

G. P. S. 
April, 1912. 



CONTENTS 



Part I — The Celestial Sphere. 

Definition of Astronomy — Fundamental Law of the 
Stars — Relations of the Earth to the Universe — ■ 
Ordinary Appearance of the Sky — The Horizon, 
the Zenith, and the Meridian — Locating the 
Stars — Altitude and Azimuth — Circular or An- 
gular Measure — Altitude Circles and Vertical 
Circles — The Apparent Motion of the Heavens 
— The North Star and Phenomena Connected 
with it — Revolution of the Stars round the Pole 
— Locating the Stars on the Celestial Sphere 
— Astronomical Equivalents of Latitude and 
Longitude — Parallels of Declination and Hour 
Circles — "The Greenwich of the Sky" — Effects 
Produced by Changing the Observer's Place on 
the Earth— The Parallel Sphere, the Right 
Sphere, and the Oblique Sphere — The Astro- 
nomical Clock — The Ecliptic — Apparent Annual 
Revolution of the Sun round the Earth— In- 
clination of the Ecliptic to the Equator: its 
Cause and its Effects — The Equinoxes — Import- 
ance of the Vernal Equinox — The Equinoctial 
Colure — The Solstices — Poles of the Ecliptic- 
Celestial Latitude and Longitude — The Zodiac 
— The Precession of the Equinoxes: its Cause 
and Effects — Revolution of the Celestial Poles — 
Past and Future Pole Stars .... 3-64 



vi Contents 



Part II — The Earth. 

Nature, Shape, and Size of the Earth — The Polar 
Compression and Equatorial Protuberance, and 
their Cause — The Attraction of Gravitation — 
The Mass of the Earth: how Found — How the 
Earth Holds the Moon, and the Sun the Planets 
— The Tides — How the Moon and Sun Produce 
Tides— Spring Tides and Neap Tides— The 
Atmosphere — The Law and Effects of Refraction 
— Dip of the Horizon — The Aberration of Light 
— Time: how Measured — Sidereal, Apparent 
Solar, and Mean Solar Time — The Clock and 
the Sun — Day and Night — Where the Days 
Begin — The Seasons — Effects of the Varying 
Declination of the Sun — Polar and Equatorial 
Day and Night — The Tropics and the Polar 
Circles — Inequality of Length of the Seasons — 
When the Seasons in the Two Hemispheres will 
be Reversed — The Calendar, the Year, and the 
Month — Reformations of the Calendar — Differ- 
ent Measures of the Month . . . 67-123 

Part III — The Solar System. 

The Sun — Distance, Size, and Condition of the 
Sun — Temperature of the Sun — Solar Heat on 
the Earth, and its Mechanical Equivalent — 
Peculiar Rotation of the Sun — Sun-spots, their 
Appearance and Probable Cause — Faculas — The 
Photosphere — Solar Prominences — Explosive 
Prominences — The Solar Corona — Parallax, and 
the Measure of Distances — Spectroscopic Analy- 
sis — How the Elements in the Sun Reveal their 
Presence — List of the Principal Solar Elements 



Contents vii 



— The Moon — Origin of the Moon — Appearance 
of its Surface — Gravity on the Moon — The 
Phases of the Moon — Causes of the Absence of a 
Lunar Atmosphere — Eclipses — How the Moon 
Causes Eclipses of the Sun — The Laws Govern- 
ing Eclipses — The Shadow during a Solar Eclipse 
— Eclipses of the Moon — Number of Eclipses in 
a Year — The Saros — The Planets — Kepler's 
Laws of Planetary Motion — Mercury — Venus — 
Mars, and its So-called Canals — Theories about 
Mars — Jupiter, its Belts and its Satellites — The 
"Great Red Spot" — Saturn, its Rings and its 
Satellites — Composition of the Rings — Uranus 
and Neptune — Comets, and the Laws of their 
Motion — Composition of Comets — The Pressure 
of Light and its Connection with Comets' Tails 
— Breaking up of Comets — Meteors and their 
Relations to Comets — The November Meteors 
and Other Celebrated Showers — Meteorites or 
Bolides which Fall upon the Earth — The Question 
of their Origin ..... 127-215 

Part IV — The Fixed Stars. 

Division of the Stars into Magnitudes — Division of 
the Stars according to their Spectra — Stars 
Larger and Smaller than the Sun — The Distances 
of the Stars — Variable Stars — Double and 
Binary Stars — Spectroscopic Binaries and how 
they are Discovered — Proper Motions ©f the 
Stars — Number of the Stars — New, or Tempo- 
rary Stars— The Milky Way— The Nebulas— 
The Two Kinds of Nebulae — Spiral Nebulas — 
The Nebular Hypothesis — Applications of Pho- 
tography to Stars and Nebulas — The Constel- 



viii Contents 

PAGES 

lations — How to Learn the Constellations — 
Their Antiquity — Description of the Principal 
Constellations Visible from the Northern Hemi- 
sphere at Various Times of the Year . 219-257 

Index 259 



ILLUSTRATIONS 



The Lunar "Crater" Copernicus Frontispiece 

Photograph of South Polar Region of the Moon 8 

The Moon near the "Crater" Tycho . . 20 

Drawing of Jupiter . . . . . .28 

Drawing of Jupiter . . . . . .28 

Jupiter ........ 38 

Saturn ........ 46 

Saturn . . . . . . . .46 

The Milky Way about Chi Cygni ... 58 

The Great Southern Star-Cluster in Centauri 64 

Photograph of a Group of Sun-Spots . . 76 

Polar Streamers of the Sun, Eclipse of 1889 88 

Solar Corona at the Eclipse of 1871 . . 88 

Morehouse's Comet, October 15, 1908 . . 96 

Morehouse's Comet, November 15, 1908 . . 96 

Head of the Great Comet of 186 i . . .104 

Halley's Comet, May 5, 1910 . . . . 104 
ix 



Illustrations 



The Six-Tailed Comet of 1744 . 
Spiral Nebula in Ursa Major (M ioi) 
The Whirlpool Nebula in Canes Venatici 
Tress Nebula (N. G. C. 6992) in Cygnus 
The Great Andromeda Nebula 
Spiral Nebula in Cepheus (H. IV. 76) 
Nebulous Groundwork in Taurus 
Nebula in Sagittarius (M. 8) . 
The Great Nebula in Orion 
Photographs of Mars 

SCHIAPARRELLI'S CHART OF MARTIAN " CANALS 



ILLUSTRATIONS IN THE TEXT 

The Rational and the Sensible Horizon 

Altitude and Azimuth 

Right Ascension and Declination 

The Ecliptic and Celestial Latitude and 
Longitude . ... 

How the Earth Controls the Moon 

The Tidal Force of the Moon . 

Refraction ..... 

Dip of the Horizon . . . . 

Sidereal and Solar Time . 



Illustrations xi 

PAGE 

The Change of Day . . . . . . 101 

The Seasons ....... 107 

Parallax of the Moon 139 

Parallax of the Sun from Transit of Venus . 141 
Spectrum Analysis . . . . . .147 

The Phases of the Moon ..... 160 

Orbits of Mars and the Earth '. . .183 

Ellipse, Parabola, and Hyperbola . . . 203 
The North Circumpolar Stars ... . . 244 

Key to North Circumpolar Stars . . . 245 



PART I. 

THE CELESTIAL SPHERE, 



PART I. 

THE CELESTIAL SPHERE. 

i. Definition of Astronomy. Astronomy 
has to do with the earth, sun, moon, planets, 
comets, meteors, stars, and nebulae; in other 
words, with the universe, or ''the aggregate 
of existing things." It is the most ancient 
of all sciences. The derivation of the name 
from two Greek words, aster, "star," and 
nomos, "law," indicates its nature. It deals 
with the law of the stars — the word "star" 
being understood, in its widest signification, 
as including every heavenly body of what- 
ever kind. The earth itself is such a body. 
Since we happen to live on the earth, it 
becomes our standpoint in space, from which 
we look out at the others. But, if we lived 
on some other planet, we would see the earth 
as a distant body in the sky, just as we now 
see Jupiter or Mars. 

Astronomy teaches us that everything in 

3 



4 THe Celestial SpHere 

the universe, from the sun and the moon 
to the most remote star or the most extra- 
ordinary nebula, is related to the earth. All 
are made of similar elementary substances 
and all obey similar physical laws. The 
same substance which is a solid upon the 
earth may be a gas or a vapour in the sun, 
but that does not alter its essential nature. 
Iron appears in the sun in the form of a hot 
vapour, but fundamentally it is the same 
substance which exists on the earth as a 
hard, tough, and heavy metal. Its different 
states depend upon the temperature to 
which it is subjected. The earth is a cool 
body, while the sun is an intensely hot 
one; consequently iron is solid on the earth 
and vaporous in the sun, just as in winter 
water is solid ice on the surface of a pond 
and steamy vapour over the boiler in the 
kitchen. Even on the earth we can make 
iron liquid in a blast furnace, and with the 
still greater temperatures obtainable in a 
laboratory we can turn it into vapour, 
thus reducing it to something like the state 
in which it regularly exists in the sun. 

This fact, that the entire universe is made 
up of similar substances, differing only in 
state according to the local circumstances 



Situation of E.artH in Heavens 5 

affecting them, is the greatest thing that 
astronomy has to tell us. It may be regarded 
as the fundamental law of the stars. 

2. The Situation of the Earth in the 
Heavens. One of the greatest triumphs 
of human intelligence is the discovery of the 
real place which the earth occupies in the 
universe. This discovery has been made in 
spite of the most deceptive appearances. 
If we accepted the sole evidence of our eyes, 
as men once did, we could only conclude that 
the earth was the centre of the universe. 
In the daytime we see the sun apparently 
moving through the sky from east to west, 
as if it were travelling in a circle round the 
earth, overhead by day and underfoot at 
night. In the night-time, we see the stars 
apparently travelling round the earth in the 
same way as the sun. The fact is, that all 
of them are virtually motionless with regard 
to the earth, and their apparent movements 
through the sky are produced by the earth's 
rotation on its axis. The earth turns round 
on itself once every twenty-four hours, 
like a spinning ball. Imagine a fly on a 
rotating school globe ; the whole room would 
appear to the fly to be revolving round it 
as the heavens appear to revolve round the 



6 THe Celestial SpKere 

earth. It would have to be a very intelligent 
insect to correct the deceptive evidence of 
its eyes. 

The actual facts, revealed by many cen- 
turies of observation and reasoning, are that 
the earth is a rotating globe, turning once 
on its axis every twenty-four hours and 
revolving once round the sun every three 
hundred and sixty-five days. The sun is 
also a globe, 1,300,000 times larger than the 
earth, but so hot that it glows with intense 
brilliance, while the substances of which 
it consists are kept in a gaseous or vaporous 
state. Besides the earth there are seven 
other principal globes, or planets, which 
revolve round the sun, at various distances 
and in various periods, and, in addition 
to these, there are hundreds of smaller 
bodies, called asteroids or small planets. 
Inhere are also many singular bodies called 
comets, and swarms of still smaller ones 
called meteors, which likewise revolve round 
the sun. 

The earth and the other bodies of which 
we have just spoken are not only cooler 
than the sun, but most of them are in a solid 
state and do not shine with light of their 
own. The sun furnishes both heat and 



Situation of Earth, in Heavens 7 

light to the smaller and cooler bodies revolv- 
ing round it. In fact, the sun is simply a 
star, resembling the thousands of other 
stars which surround us in the sky, and 
its apparent superiority to them is due only 
to the fact that it is relatively near-by while 
they are far away. It is probable that all, 
or most, of the stars also have planets, 
comets, and meteors revolving round them, 
but invisible owing to their immense distance. 
The "paths" in which the earth and the 
other planets and bodies travel in their 
revolutions round the sun are called their 
orbits. These orbits are all elliptical in 
shape, but those of the earth and the other 
large planets are not very different from 
circles. Some of the asteroids, and all of 
the comets, however, travel in elliptical 
orbits of considerable eccentricity, i. e., 
which differ markedly from circles. The 
orbit of the earth differs so slightly from a 
circle that the eccentricity amounts to only 
about one-sixtieth. The distance of the earth 
from the sun being, on the average, 93,000,000 
miles, the eccentricity of its orbit causes it 
to approach to within about 91,500,000 
miles in winter (of the northern hemisphere) 
and to recede to about 94,500,000 miles 



8 THe Celestial SpKere 

in summer. The point in its orbit where 
the earth is nearest the sun is called peri- 
helion, and the point where it is farthest 
from the sun, aphelion. The earth is at 
perihelion about Jan. i, and at aphelion 
about July 4. 

Now, in order to make a general picture in 
the mind of the earth's situation, let the 
reader suppose himself to be placed out in 
space as far from the sun as from the other 
stars. Then, if he could see it, he would 
observe the earth as a little speck, shining 
like a mote in the sunlight, and circling in 
its orbit close around the sun. The universe 
would appear to him to be somewhat like an 
immense spherical room filled with scattered 
electric-light bulbs, suspended above, below, 
and all around him, each of these bulbs repre- 
senting a sun, and if there were minute insects 
flying around each light, these insects would 
represent the planets belonging to the 
various suns. One of the glowing bulbs 
among the multitude would stand for our 
sun, and one of the insects circling round 
it would be the earth. 

We have already remarked that the 
rotation of the earth on its axis causes all 
the other heavenly bodies to appear to revolve 



Photograph of the South Polar Region of the Moon 

Made by G. W. Ritchey with the forty-inch refractor of the Yerkes 
Observatory. 



Horizon, ZenitK, and Meridian 9 

round it once every twenty-four hours, and 
we must now add that the earth's revolution 
round the sun causes the same bodies to 
appear to make another, slower revolution 
round it once every year. This introduces 
a complication of apparent motions which it 
is the business of astronomy to deal with, 
and which we shall endeavour to explain. 

3. The Horizon, the Zenith, and the 
Meridian. First, let us consider what is the 
ordinary appearance of the sky. When we 
go out of doors on a clear night we see the 
heavens in the shape of a great dome arched 
above us and filled with stars. What we 
thus see is one half of the spherical shell 
of the heavens which surrounds us on all 
sides, the earth being apparently placed at 
its centre. The other half is concealed from 
our sight behind, or below, the earth. This 
spherical shell, of which only one half is 
visible to us at a time, is called the celestial 
sphere. Now, the surface of the earth 
seems to us (for this is another of the 
deceptive appearances which astronomy has 
to correct) to be a vast flat expanse, whose 
level is broken by hills and mountains, 
and the visible half of the celestial sphere 
seems to bend down on all sides and to rest 



<m 



io The Celestial SpHere 

upon the earth in a circle which extends all 
around us. This circle, where the heavens 
and the earth appear to meet, is called the 
horizon. As we ordinarily see it, the horizon 
appears irregular and broken on account of 
the unevenness of the earth's surface, but if 
we are at sea, or in the midst of a great level 
prairie, the horizon appears as a smooth 
circle, everywhere equally distant from the 
eye. This circle is called the sensible horizon. 
But there is another, ideal, horizon, used in 
astronomy, which is called the rational 
horizon. It is of the utmost importance 
that we should clearly understand what is 
meant by the rational horizon, and for this 
purpose we must consider another fact con- 
cerning the dome of the sky. 

We now turn our attention to the centre 
of that dome, which, of course, is the point 
directly overhead. This point, which is 
of primary importance, is called the zenith. 
The position of the zenith is indicated by 
the direction of a plumb-line. If we imagine 
a plumb-line to be suspended from the 
centre of the sky overhead, and to pass 
into the earth at our feet, it would run through 
the centre of the earth, and, if it were con- 
tinued onward in the same direction, it 



Horizon, ZenitH, and Meridian II 

would, after emerging from the other side of 
the earth, reach the centre of the invisible 
half of the sky-dome at a point diametrically 
opposite to the zenith. This central point 
of the invisible half of the celestial sphere, 
lying under our feet, is called the nadir. 

Keeping in mind the definitions of zenith 
and nadir that have just been given, we are 
in a position to understand what the rational 
horizon is. It is a great circle whose plane 
cuts through the centre of the earth, and 
which is situated exactly half-way between 
the zenith and the nadir. This plane is 
necessarily perpendicular, or at right angles, 
to the plumb -line joining the zenith and the 
nadir. In other words, the rational horizon 
divides the celestial sphere into two precisely 
equal halves, an upper and a lower half. 
In a hilly or mountainous country the sensible 
or visible horizon differs widely from the 
rational, or true horizon, but at sea the two 
are nearly identical. This arises from the 
fact, that the earth is so excessively small in 
comparison with the distances of most of the 
heavenly bodies that it may be regarded as 
a mere point in the midst of the celestial 
sphere. 

Besides the horizon and the zenith there 



12 THe Celestial SpKere 

is one other thing of fundamental importance 
which we must learn about before proceed- 
ing further, — the meridian. The meridian 



Fig. i. The Rational and the Sensible Horizon. 

Let C be the earth's centre, O the place of the observer, 
and H D the rational horizon passing through the centre 
of the earth. For an object situated near the earth, as 
at A, the sensible horizon makes a large angle with the 
rational horizon. If the object is farther away, as at B, 
the angle becomes less; and still less, again, if the object 
is at D. It is evident that if the object be immensely 
distant, like a star, the sensible horizon O S will be 
practically parallel with the rational horizon, and will 
blend with it, because the radius, or semi-diameter, of 
the earth, O C, is virtually nothing in comparison with 
the distance of the star. 

is an imaginary line, or semicircle, beginning 
at the north point on the horizon, running 
up through the zenith, and then curving 
down to the south point. It thus divides the 
visible sky into two exactly equal halves, 
an eastern and a western half. In the 
ordinary affairs of life we usually think only 
of that part of the meridian which extends 
from the zenith to the south point on the 
horizon (which is sometimes called the 
"noon-line" because the sun crosses it at 
noon), but in astronomy the northern half 



Altitude and AzimxitH 13 

of the meridian is as important as the 
southern. 

4. Altitude and Azimuth. Now, suppose 
that we wish to indicate the location of a 
star, or other object, in the sky. To do so, 
we must have some fixed basis of reference, 
and such a basis is furnished by the horizon 
and the zenith. If we tried to describe the 
position of a star, the most natural thing 
would be, first, to estimate, or measure, 
its height above the horizon, and, second, to 
indicate the direction in which it was situated 
with regard to the points of the compass. 
These two measures, if they were accurately 
made, would enable another person to find 
the star in the sky. And this is precisely 
what is done in astronomy. The height 
above the horizon is called altitude, and the 
bearing with reference to the points of the 
compass is called azimuth. Together these 
are known as co-ordinates. In order to 
systematise this method of measuring the 
location of a star, the astronomer uses ima- 
ginary circles drawn on the celestial sphere. 
The horizon and the meridian are two of 
these circles. In addition to these, other 
imaginary circles are drawn parallel to the 
horizon and becoming smaller and smaller 



14 



THe Celestial Sphere 



until the uppermost one may run close round 
the zenith, which is the common centre of 




e. zro a 

Altitude and Azimuth. 



C is the place of the observer. 

N C S, a north-and-south line drawn in the plane of the 

horizon. 

E C W, an east-and-west line in the plane of the horizon. 

N E S W, the circle of the horizon. 

Z, the observer's zenith. 

N Z S, vertically above N C S, the meridian. 

EZW, the prime vertical. 

Zss', part of a vertical circle drawn through the star s. 

The circle through s parallel to the horizon is an altitude 

circle. 

The angle s C s\ or the arc s' s, represents the star's 

altitude. 

The angle s C Z, or the arc Z s, is the star's zenith 

distance. 

To find the azimuth, the angular distance round the 

horizon from S (o°), through W, N, E, to the point 

where the star's vertical circle meets the horizon, is 

measured. In this case it is 31 5 . But if we measured 

it eastward from the south point it would be — 45 . 



the entire set. These are called altitude 
circles, because each one throughout its, 



Altitude and Azimuth. 15 

whole extent is at an unvarying height, or 
altitude, above the horizon. Such circles 
may be drawn anywhere we please, so as to 
pass through any chosen star or stars. If 
two stars in different quarters of the sky 
are found to lie on the same circle, then we 
know that both have the same altitude. 

Then another set of circles is drawn per- 
pendicular to the horizon, and all intersecting 
at the zenith and the nadir. These are 
called vertical circles, from the fact that they 
are upright to the horizon. That m one of 
the vertical circles which cuts the horizon 
at the north and south points coincides 
with the meridian, which we have already 
described. The vertical circle at right 
angles to the meridian is called the prime 
vertical. It cuts the horizon at the east and 
west points, dividing the visible sky into a 
northern and a southern half. Like the 
altitude circles, vertical circles may be drawn 
anywhere we please so as to pass through a 
star in any quarter of the sky — but the 
meridian and the prime vertical are fixed. 

With the two sets of circles that have 
just been described, it is possible to indicate 
accurately the location of any heavenly 
body, at any particular moment. Its al- 



16 The Celestial Sphere 

titude is ascertained by measuring, along the 
vertical circle passing through it, its distance 
from the horizon. (Sometimes it is con- 
venient to measure, instead of the altitude 
of a star, its zenith distance, which is also 
reckoned on the vertical circle.) 

To ascertain the azimuth, we must first 
choose a point of beginning on the horizon. 
Any of the cardinal points, i.e., east, west, 
north, or south, may be employed for this 
purpose, but in astronomy it is customary 
to use only the south point, and to carry the 
measure westward all round the circle of the 
horizon, and so back to the point of begin- 
ning in the south. This involves circular, 
or degree, measure, to which a few words 
must now be devoted. 

Every circle, no matter how large or how 
small, is divided into 360 equal parts, called de- 
grees, usually indicated by the sign (°) ; each 
degree is subdivided into 60 equal parts called 
minutes, indicated by the sign (') ; and each 
minute is subdivided into 60 equal parts 
called seconds, indicated by the sign ("). 
Thus there are 360 , or 21,600', or 1,296,000" 
in every complete circle. The actual length 
of a degree in inches, yards, or miles, depends 
upon the size of the circle, but no circle ever 



Altitude and Azimuth. 17 

has more than 360 °, and a degree of any 
particular circle is precisely equal to any 
other degree of that same circle. Thus, if 
a circle is 360 miles in circumference, every 
one of its degrees will be one mile long. 
In mathematics, a degree usually means 
not a distance measured along the circum- 
ference of a circle, but an angle formed at 
the centre of the circle between two lines 
called radii (radius in the singular), which 
lines, where they intersect the circumference, 
are separated by a distance equal to one 
360th of the entire circle. But, for ordinary 
purposes, it is simpler to think of a degree 
as an arc equal in length to one 360th of 
the circle. Now, since the horizon, and 
the other imaginary lines drawn in the sky, 
are all circles, it is evident that the principle 
of circular measure may be applied to them, 
and indeed must be so applied in order that 
they shall be of use to us in indicating the 
position of a star. 

To return, then, to the measurement of 
the azimuth of a star. Since the south 
point is the place of beginning, we mark it 
o°, and we divide the circle of the horizon 
into 360 , counting round westward. Sup- 
pose we see a star somewhere in the south- 



l8 TTHe Celestial Sphere 

western quarter of the sky; then the point 
where the vertical circle passing through 
that star intersects the horizon will indicate 
its azimuth. Suppose that this point is 
found to be 25 west of south; then 25 will 
be the star's azimuth. Suppose it is 90 ; 
then the azimuth is 90 , and the star must 
be on the prime vertical in the west, because 
west, being one quarter of the way round the 
horizon from south, is 90 ° in angular distance 
from the south point. Suppose the azimuth 
is 180 ; then the star must be on the meridian 
north of the zenith, because north is exactly 
half-way, or 180 round the horizon from the 
south point. Suppose the azimuth is 270 ; 
then the star must be on the prime vertical 
in the east, because east is 270 , or three 
quarters of the way round from the south 
point. If the star is on the meridian in 
the south its azimuth may be called either 
o° or 360 , because on any graduated circle 
the mark indicating 360 coincides in position 
with o°, that being at the same time the 
point of beginning and the point of ending. 
The same system of angular measure is 
applied in ascertaining a star's altitude. 
Since the horizon is half-way between the 
zenith and the nadir it must be just 90 



Apparent Motion, of tHe Heavens 19 

from either. If a star is in the zenith, then 
its altitude is 90 , and if it is below the zenith 
its altitude lies somewhere between o° and 
90 . In any case it cannot be less than o° 
nor more than 90 °. Having measured the 
altitude and the azimuth we have the two 
co-ordinates which are needed to indicate 
accurately the place of a star in the sky. 
But, as we shall see in a moment, other 
co-ordinates beside altitude and azimuth 
are needed for a complete description of the 
places of the stars on the celestial sphere. 
Owing to the apparent revolution of the 
heavens round the earth, the altitudes and 
azimuths of the celestial bodies are continu- 
ally changing. We shall now study the causes 
of these changes. 

5. The Apparent Motion of the Heavens. 
We have likened the earth to a rotating 
school globe. As such a globe turns, any 
particular spot on it is presented in succession 
toward the various sides of the room. In 
precisely the same way any spot on the earth 
is turned by its rotation successively toward 
various parts of the surrounding sky. To 
understand the effect of this, a little patient 
watching of the actual heavens will be 
required, but this has the charm of all out- 



20 THe Celestial Sphere 

of-doors observation of nature, and it will 
be found of fascinating interest as the facts 
begin to unfold themselves. 

It is best to begin by finding the North 
Star, or pole star. If you are living not far 
from latitude 40 ° north, which is the median 
latitude of the United States, you must, 
after determining as closely as you can the 
situation of the north point, look upward 
along the meridian in the north until your 
eyes are directed to a point about 40 above 
the horizon. Forty degrees is somewhat 
less than half-way from the horizon to the 
zenith, which, as we have seen, are separated 
by an arc of 90 . At that point you will 
notice a lone star of what astronomers call 
the second magnitude. This is the cele- 
brated North Star. It is the most useful to 
man of all the stars, except the sun, and it 
differs from all the others in a way presently 
to be explained. But first it is essential 
that you should make no mistake in identi- 
fying it. There are certain landmarks in 
the sky which make such identification 
certain. In the first place, it is always so 
close to the meridian in the north, that by 
naked-eye observation you would probably 
never suspect that it was not exactly on the 




The Moon Near the "Crater" Tycho 

Photographed at the Lick Observatory under the direction of E. S. Holden. 
Tycho is the regular oval depression a little below the centre of the view. 
The vast depression, 140 miles across, with a row of smaller craters within, 
below the centre of the view at the top, is Clavius. The photograph was 
made when sundown was approaching on that part of the moon. Observe 
the jagged line of advancing night lying across the rugged surface on the 
western (left-hand) side. 



Apparent Motion of tHe Heavens 21 

meridian. Then, its altitude is always equal, 
or very nearly equal, to the latitude of the 
place where you happen to be on the earth, 
so that if you know your latitude you know 
how high to carry your eye above the north- 
ern horizon. If you are in latitude 50 , 
the star will be at 50 altitude, and if your 
latitude is 30 , the altitude of the star will 
be 30 . Next you will notice that the North 
Star is situated at the end of the handle 
of a kind of dipper-shaped figure formed by 
stars, the handle being bent the wrong way. 
All of the stars forming this " dipper' ' are 
faint, except the two which are farthest from 
the North Star, in the outer edge of the bowl, 
one of which is about as bright as the North 
Star itself. Again, if you carry your eye 
along the handle to the bowl, and then 
continue onward about as much farther, 
you will be led to another, larger, more 
conspicuous, and more perfect, dipper- 
shaped figure, which is in the famous con- 
stellation of Ursa Major, or the Great Bear. 
This striking figure is called the Great 
Dipper (known in England as The Wain). 
It contains seven conspicuous stars, all 
of which, with one exception, are equal in 
brightness to the North Star. Now, look 



22 THe Celestial SpKere 

particularly at the two stars which indicate 
the outer side of the bowl of this dipper, 
and you will find that if you draw an imagin- 
ary line through them toward the meridian 
in the north, it will lead your eye directly 
back to the North Star. These two sig- 
nificant stars are often called The Pointers. 
With their aid you can make sure that you 
have really found the North Star. 

Having found it, begin by noting the 
various groups of stars, or constellations, 
in the northern part of the sky, and, as the 
night wears on, observe whether any change 
takes place in their position. To make our 
description more definite, we will suppose 
that the observations begin at nine o'clock 
p.m. about the ist of July. At that hour 
and date, and from the middle latitudes of 
the United States, the Great Dipper is seen 
in a south-westerly direction from the North 
Star, with its handle pointing overhead. 
At the same time, on the opposite side of the 
North Star, and low in the north-east, 
appears the remarkable constellation of 
Cassiopeia, easily recognisable by a zigzag 
figure, roughly resembling the letter W, 
formed by its five principal stars. Fix the 
relative positions of these constellations in 



Apparent Motion of tKe Heavens 23 

the memory, and an hour later, at 10 P.M., 
look at them again. You will find that they 
have moved, the Great Dipper sinking, while 
Cassiopeia rises. Make a third observation 
at 11 p.m., and you will perceive that the 
motion has continued, The Pointers having 
descended in the north-west, until they are 
on a level with the North Star, while Cas- 
siopeia has risen to nearly the same level 
in the north-east. In the meantime, the 
North Star has remained apparently motion- 
less in its original position. If you repeat 
the observation at midnight, you will 
find that the Great Dipper has descended 
so far that its centre is on a level with 
the North Star, and that Cassiopeia has 
proportionally risen in the north-east. It 
is just as if the two constellations were 
attached to the ends of a rod pivoted at the 
centre upon the North Star and twirling 
about it. 

In the meantime you will have noticed 
that the figure of the "Little Dipper," 
attached to the North Star, which had its 
bowl toward the zenith at 9 p.m., has swung 
round so that at midnight it is extended 
toward the south-west. Thus, you will 
perceive that the North Star is like a hub 



24 THe Celestial Sphere 

round which the heavens appear to turn, 
carrying the other stars with them. 

To convince yourself that this motion is 
common to the stars in all parts of the sky, 
you should also watch the conduct of those 
which pass overhead, and those which are 
in the southern quarter of the heavens. For 
instance, at 9 p.m. (same date), you will 
see near the zenith a beautiful coronet 
which makes a striking appearance although 
all but one of its stars are relatively faint. 
This is the constellation Corona Borealis, 
or the Northern Crown. As the hours pass 
you will see the Crown swing slowly west- 
ward, descending gradually toward the 
horizon, and if you persevere in your ob- 
servations until about 5 a.m., you will see 
it set in the north-west. The curve that it 
describes is concentric with those followed 
by the Great Dipper and Cassiopeia, but, 
being farther from the North Star than they 
are, and at a distance greater than the 
altitude of the North Star, it sinks below 
the horizon before it can arrive at a point 
directly underneath that star. Then take 
a star far in the south. At 9 o'clock, you 
will perceive the bright reddish star Ant ares, 
in the constellation Scorpio, rather low in 



Apparent Motion of tHe Heavens 25 

the south and considerably east of the 
meridian. Hour after hour it will move 
westward, in a curve larger than that of the 
Northern Crown, but still concentric with it. 
A little before 10 p.m., it will cross the 
meridian, and between 1 and 2 A.M., it 
will sink beneath the horizon at a point south 
of west. 

So, no matter in what part of the heavens 
you watch the stars, you will see not only 
that they move from east to west, but that 
this motion is performed in curves concentric 
round the North Star, which alone appears 
to maintain its place unchanged. Along 
the eastern horizon you will perceive stars 
continually rising; in the middle of the sky 
you will see others continually crossing the 
meridian — a majestic march of constellations, 
— and along the western horizon you will find 
still others continually setting. If you could 
watch the stars uninterruptedly through- 
out the twenty-four hours (if daylight did 
not hide them from sight during half that 
period), you would perceive that they go 
entirely round the celestial sphere, or rather 
that it goes round with them, and that at 
the end of twenty-four hours they return to 
their original positions. But you can do this 



26 THe Celestial SpHere 

just as well by looking at them on two suc- 
cessive nights, when you will find that at the 
same hour on the second night they are back 
again, practically in the places where you 
saw them on the first night. 

Of course, what happens on a July night 
happens on any other night of the year. 
We have taken a particular date merely 
in order to make the description clearer. 
It is only necessary to find the North Star, 
the Great Dipper, and Cassiopeia, and you 
can observe the apparent revolution of the 
heavens at any time of the year. These 
constellations, being so near the North 
Star that they never go entirely below the 
horizon in middle northern latitudes, are 
always visible on one side or another of the 
North Star. 

Now, call upon your imagination to deal 
with what you have been observing, and you 
will have no difficulty in explaining what 
all this apparent motion of the stars means. 
You already know that the heavens form 
a sphere surrounding the earth. You have 
simply to suppose the North Star to be 
situated at, or close to, the north end, or 
north pole, of an imaginary axle, or axis, 
round which the celestial sphere seems to 



Apparent Motion of trie Heavens 27 

turn, and instantly the whole series of 
phenomena will fall into order, and the 
explanation will stare you in the face. That 
explanation is that the motion of all the 
stars in concentric circles round the North 
Star is due to an apparent revolution of 
the whole celestial sphere, like a huge 
hollow ball, about an axis, the position of 
one of whose poles is graphically indicated 
in the sky by the North Star. The cir- 
cles in which the stars seem to move are 
perpendicular to this axis, and inclined 
to the horizon at an angle depending 
upon the altitude of the North Star at the 
place on the earth where the observations 
are made. 

Another important fact demands our 
attention, although the thoughtful reader 
will already have guessed it — the north pole 
of the celestial sphere, whose position in the 
sky is closely indicated by the North Star, 
is situated directly over the north pole 
of the earth. This follows from the fact that 
the apparent revolution of the celestial 
sphere is due to the real rotation of the earth. 
You can see that the two poles, that of the 
earth and that of the heavens, must neces- 
sarily coincide, by taking a school globe and 



28 THe Celestial SpHere 

imagining that you are an intelligent little 
being dwelling on its surface. As the globe 
turned on its axis you would see the walls of 
the room revolving round you, and the poles 
of the apparent axis round which the room 
turned, would, evidently, be directly over 
the corresponding poles of the globe itself. 
Another thing which you could make clear 
by this experiment is that, as the poles of 
the celestial sphere are over the earth's 
poles, so the celestial equator, or equator 
of the heavens, must be directly over the 
equator of the earth. 

We can determine the location of the 
poles of the heavens by watching the revo- 
lution of the stars around them, and we can 
fix the position of the circle of the equator 
of the heavens, by drawing an imaginary 
line round the celestial sphere, half-way 
between the two poles. We have spoken 
specifically only of the north pole, but, of 
course, there is a corresponding south pole 
situated over the south pole of the earth, 
but whose position is invisible from the 
northern hemisphere. It happens that the 
place of the south celestial pole is not in- 
dicated to the eye, like that of the northern, 
by a conspicuous star. 




Drawing of Jupiter 




Drawing of Jupiter 

Note the change of details between the two drawings, made at different 
times. Similar changes are continually occurring. 



Locating tKe Stars 29 

6. Locating the Stars on the Celestial 
Sphere. Having found the poles and the 
equator of the celestial sphere, we begin 
to see how it is possible to make a map, 
or globe, of the heavens just as we do of the 
earth, on which the objects that they contain 
may be represented in their proper positions. 
When we wish to describe the location of 
an object on the earth, a city for instance, 
we have to refer to a system of imaginary 
circles, drawn round the earth and based 
upon the equator and the poles. These 
circles enable us to fix the place of any point 
on the earth with accuracy. One set of 
circles called parallels of latitude are drawn 
east and west round the globe parallel to 
the equator, and becoming smaller and 
smaller until the smallest runs close round 
their common central point, which is one 
of the poles. Each pole of the earth is the 
centre of such a set of circles all parallel to 
the equator. Since each circle is unvarying 
in its distance from the equator, all places 
which are situated anywhere on that circle 
have the same latitude, or distance from the 
equator, either north or south. 

But to know the latitude of any place on 
the earth is not sufficient; we must also 



30 THe Celestial SpHere 

know what is called its longitude, or its 
angular distance east or west of some chosen 
point on the equator. This knowledge is 
obtained with the aid of another set of circles 
drawn north and south round the earth, 
and all meeting and crossing at the poles. 
These are called meridians of longitude. 
In order to make use of them we must, 
as already intimated, select some particular 
meridian whose crossing point on the equator 
will serve as a place of beginning. By 
common consent of the civilised world, the 
meridian which passes through the obser- 
vatory at Greenwich, near London, has been 
chosen for this purpose. It is, like all the 
meridians, perpendicular to the equator, 
and it is called the prime meridian of the 
earth. 

In locating any place on the earth, then, 
we ascertain by means of the parallel of 
latitude passing through it how far in 
degrees, it is north or south of the equator, 
and by means of its meridian of longitude 
how far it is east or west of the prime me- 
ridian, or meridian of Greenwich. These 
two things being known, we have the exact 
location of the place on the earth. Let us 
now see how a similar system is applied to 



Locating tKe Stars 31 

ascertain the location of a heavenly body 
on the celestial sphere. 

We have observed that the poles of the 
heavens correspond in position, or direction, 
with those of the earth, and that the equator 
of the heavens runs round the sky directly 
over the earth's equator. It follows that 
we can divide the celestial sphere just as 
we do the surface of the earth by means of 
parallels and meridians, corresponding to the 
similar circles of the earth. On the earth, 
distance from the equator is called latitude, 
and distance from the prime meridian, 
longitude. In the heavens, distance from 
the equator is called declination, and distance 
from the prime meridian, right ascension; 
but they are essentially the same things 
as latitude and longitude, and are measured 
virtually in the same way. In place of 
parallels of latitude, we have on the celestial 
sphere circles drawn parallel to the equator 
and centring about the celestial poles, 
which are called parallels of declination, 
and in place of meridians of longitude, we 
have circles perpendicular to the equator, 
and drawn through the celestial poles, 
which are called hour circles. The origin 
of this name will be explained in a moment. 



32 THe Celestial SpHere 

For the present it is only necessary to fix 
firmly in the mind the fact that these two 
systems of circles, one on the earth and the 
other in the heavens, are fundamentally 
identical. 

Just as on the earth geographers have 
chosen a particular place, viz. Greenwich, 
to fix the location of the terrestrial prime 
meridian, so astronomers have agreed upon 
a particular point in the heavens which 
serves to determine the location of the 
celestial prime meridian. This point, which 
lies on the celestial equator, is known as the 
vernal equinox. We shall explain its origin 
after having indicated its use. The hour 
circle which passes through the vernal 
equinox is the prime meridian of the heavens, 
and the vernal equinox itself is sometimes 
called the ''Greenwich of the Sky." 

If, now, we wish to ascertain the exact 
location of a star on the celestial sphere, as 
we would that of New York, London, or 
Paris, on the earth, we measure along the hour 
circle running through it, its declination, 
or distance from the celestial equator, and 
then, along its parallel of declination, we 
measure its right ascension, or distance 
from the vernal equinox. Having these 



After SHiftingf Observer's Position 33 

two co-ordinates, we possess all that is nec- 
essary to enable us to describe the position 
of the star, so that someone else looking for 
it, may find it in the sky, as a navigator 
finds some lone island in the sea by knowing 
its latitude and longitude. 

Declination, as' we have seen, is simply 
another name for latitude, but right ascen- 
sion, which corresponds to longitude, needs a 
little additional explanation. It differs from 
longitude, first, in that, instead of being reck- 
oned both east and west from the prime 
meridian, it is reckoned only toward the east, 
the reckoning being continued uninterrup- 
tedly entirely round the circle of the 
equator; and, second, in that it is usually 
counted not in degrees, minutes, and seconds 
of arc, but in hours, minutes, and seconds 
of time. The reason for this is that, 
since the celestial sphere makes one com- 
plete revolution in twenty-four hours, it 
is convenient to divide the circuit into 
twenty-four equal parts, each correspond- 
ing to the distance through which the 
heavens appear to turn in one hour. This 
explains the origin of the term hour circles 
applied to the celestial meridians, which, 

by intersecting the equator, divide it into 
3 



34 THe Celestial SpKere 

twenty-four equal parts, each part corre- 
sponding to an hour of time. In ex- 
pressing right ascension in time, the Roman 
numerals — i, n, in, iv, v, vi, vn, vm, 

IX, X, XI, XII, XIII, XIV, XV, XVI, XVII, 
XVIII, XIX, XX, XXI, XXII, XXIII, xxiv — 

are employed for the hours, and the letters 
m and s respectively for the minutes and 
seconds. Since there are 360 in every 
circle, it is plain that one hour of right ascen- 
sion corresponds to 15 . So, too, one minute 
of right ascension corresponds to 15', and 
one second to 15". It will be found useful 
to memorise these relations. 

7. Effects Produced by Changing the 
Observer's Place on the Earth. The reader 
will recall that in Sect. 4 we described an- 
other system of circles for determining the 
places of stars, a system based on the horizon 
and the zenith. This horizon-zenith system 
takes no account of the changes produced 
by the apparent motion of the heavens, 
and consequently it is not applicable to 
determining the absolute positions of the 
stars on the celestial sphere. It simply 
shows their positions in the visible half of 
the sky, as seen at some particular time 
from some definite point on the earth. In 



After Shifting Observer's Position 35 




Fig. 3. Right Ascension and Declination. 

The plane of the horizon, with the north, south, east, and 

west points, and the zenith, are represented as in Fig. 2. 

P and P' are the poles of the celestial sphere, the dotted 

line connecting them representing the direction of the 

axis, both of celestial sphere and the earth. 

The circle Eq Eq' is the equator. 

V is the vernal equinox, or the point on the equator 

whence right ascension is reckoned round toward the 

east. 

The circle passing through s, parallel to the equator, is a 

declination circle. 

The circle P s P' is the hour circle of the star s. 

The arc of this hour circle contained between s and the 

point where it meets the equator is the star's declination. 

Its right ascension is measured by the arc of the equator 

contained between V and the point where its hour circle 

meets the equator, or by the angle V P s. 

The hour circle P V P', passing through the vernal 

equinox, is the equinoctial colure. When this has 

moved up to coincidence with the meridian, N Z S, it 

will be astronomical noon. 



36 THe Celestial SpHere 

order to show the changing relations of this 
system to that which we have just been de- 
scribing, let us consider the effects produced 
by shifting our place of observation on the 
earth. Since the zenith is the point overhead 
and the nadir the point underfoot, and the 
horizon is a great circle drawn exactly half- 
way between the zenith and the nadir, it 
is evident, upon a moment's consideration, 
that every place on the earth has its own 
zenith and its own horizon. It is also clear 
that every place must have its own meridian, 
because the meridian is a north and south 
line running directly over the observer's 
head. You can see how this is, if you reflect 
that for an observer situated on the other 
side of the earth what is overhead for you 
will be underfoot for him, and vice versa. 
Thus the direction of our zenith is the 
direction of the nadir for our antipodes, 
and the direction of their zenith is the 
direction of our nadir. They see the half 
of the sky which is invisible to us, and we 
the half which is invisible to them. 

Now, suppose that we should go to the 
north pole. The celestial north pole would 
then be in our zenith, and the equator would 
correspond with the horizon. Thus, for 



After SHiftingJ Observer's Position 37 

an observer at the north pole the two systems 
of circles that we have described would fall 
into coincidence. The zenith would cor- 
respond with the pole of the heavens; the 
horizon would correspond with the celestial 
equator ; the vertical circles would correspond 
with the hour circles; and the altitude 
circles would correspond with the circles of 
declination. The North Star, being close 
to the pole of the heavens, would appear 
directly overhead. Being at the zenith, 
its altitude would be 90 (see Sect. 4). 
Peary, if he had visited the pole during the 
polar night, would have seen the North 
Star overhead, and it would have enabled 
him with relatively little trouble to deter- 
mine his exact place on the earth, or, in other 
words, the exact location of the north 
terrestrial pole. With the pole star in the 
zenith, it is evident that the other stars 
would be seen revolving round it in circles 
parallel to the horizon. All the stars 
situated north of the celestial equator would 
be simultaneously and continuously visible. 
None of them would either rise or set, but 
all, in the course of twenty-four hours, would 
appear to make a complete circuit horizontally 
round the sky. This polar presentation of the 



38 THe Celestial SpHere 

celestial sphere is called the parallel sphere, 
because the stars appear to move parallel 
to the horizon. No man has yet beheld 
the nocturnal phenomena of the parallel 
sphere, but if in the future some explorer 
should visit one of the earth's poles during 
the polar night, he would behold the spectacle 
in all its strange splendour. 

Next, suppose that you are somewhere 
on the earth's equator. Since the equator 
is everywhere 90 , or one quarter of a circle, 
from each pole, it is evident that looking at 
the sky from the equator you would see the 
two poles (if there was anything to mark 
their places) lying on the horizon one exactly 
in the north and the other exactly in the 
south. The celestial equator would cor- 
respond with the prime vertical, passing 
east and west directly over your head, and 
all the stars would rise and set perpendicu- 
larly to the horizon, each describing a semi- 
circle in the sky in the course of twelve 
hours. During the other twelve hours, the 
same stars would be below the horizon. Stars 
situated near either of the poles would de- 
scribe little semi-circles near the north or 
the south point; those farther away would 
describe larger semi-circles; those close to 




Jupiter 

Photographed at the Lick Observatory. 
Observe on the left the Great Red Spot, which first appeared in 1878. 



After SHifting Observer's Position 39 

the celestial equator would describe semi- 
circles passing overhead. But all, no matter 
where situated, would describe their visible 
courses in the same period of time. This 
equatorial presentation of the celestial sphere 
is called the right sphere, because the stars 
rise and set at a right angle to the plane of 
the horizon. Comparing it with the system 
of circles on which right ascension and de- 
clination are based, we see that, as the prime 
vertical corresponds with the celestial equa- 
tor, so the horizon must represent an hour 
circle. The meridian also represents an hour 
circle. It may require a little thought to 
make this clear, but it will be a good 
exercise. 

Finally, if you are somewhere between the 
equator and one of the poles, which is the 
actual situation of the vast majority of man- 
kind, you see either the north or the south 
pole of the heavens elevated to an altitude 
corresponding with your latitude, and the 
stars apparently revolving round it in circles 
inclined to the horizon at an angle depending 
upon the latitude. The nearer you are to 
the equator, the steeper this angle will be. 
This ordinary presentation of the celestial 
sphere is called the oblique sphere. Its 



40 XKe Celestial SpKere 

horizon does not correspond with either the 
equator or the prime vertical, and its zenith 
and nadir lie at points situated between the 
celestial poles and the celestial equator. 

8. The Astronomical Clock and the Eclip- 
tic. It will be remembered that the meridian 
of any place on the earth is a straight north 
and south line running through the zenith 
and perpendicular to the horizon. More 
strictly speaking, the meridian is a circle 
passing from north to south directly overhead 
and corresponding exactly with the meridian 
of longitude of the place of observation. 
Now, let us consider the hour circles on the 
celestial sphere. They are drawn in the 
same way as the meridians on the earth. 
But the celestial sphere appears to revolve 
round the earth, and as it does so it must 
carry the hour circles with it, since they are 
fixed in position upon its surface. Fix 
your attention upon the first of these hour 
circles, i. e., the one which runs through the 
vernal equinox. Its right ascension is called 
o hours, because it is the starting point. 
Suppose that at some time we find the vernal 
equinox exactly in the south; then the o 
hour circle, or the prime meridian of the 
heavens, will, at that instant, coincide with 



Astronomical ClocK and Ecliptic 41 

the meridian of the place of observation. 
But one hour later, in consequence of the 
motion of the heavens, the vernal equinox, 
together with the circle of O hours, will be 
1 5 , or one hour of right ascension, west 
of the meridian, and the hour circle marked 
1 will have come up to, and for an instant will 
be blended with, the meridian. An hour 
later still, the circle of 11 hours right ascension 
will have taken its place on the meridian, 
while the vernal equinox and the circle of o 
hours will be 11 hours, or 30 °, west of the 
meridian. And so on, throughout the entire 
circuit of the sky. 

What has just been said makes it evident 
that the apparent motion of the heavens 
resembles the movement of a clock, the 
vernal equinox, or the circle of o hours, 
serving as a hand, or pointer, on the dial. 
Astronomers use it in exactly that way, for 
astronomical clocks are made with dials 
divided into twenty-four hour spaces, and the 
time reckoning runs continuously from o 
hours to xxiv hours. The "astronomical 
day " begins when the vernal equinox is on the 
meridian. At that instant the hands of the 
astronomical clock mark o hours, o minutes, 
o seconds. Thus the clock follows the motion 



42 THe Celestial SpHere 

of the heavens, and the astronomer can tell 
by simply glancing at the dial, and without 
looking at the sky, where the vernal equinox 
is, and what is the right ascension of any 
body which may at that moment be on the 
meridian. 

We must now explain a little more fully 
what the vernal equinox is, and why it has 
been chosen as the "Greenwich of the Sky." 
Its position is not marked by any star, but 
is determined by means of the intersecting 
circles that we have described. There is 
one other such circle, that we have not yet 
mentioned, which bears a peculiar relation 
to the vernal equinox. This is the ecliptic. 
Just as the daily, or diurnal, rotation of the 
earth on its axis causes the whole celestial 
sphere to appear to make one revolution 
every day, so the yearly or annual revolution 
of the earth in its orbit about the sun causes 
the sun to appear to make one revolution 
through the sky every year. As the earth 
moves onward in its orbit, the sun seems to 
move in the opposite direction. Inasmuch 
as there are 360 in a complete circle and 
365 days in a year, the apparent motion of 
the sun amounts to nearly i° per day, or 
30 per month. In twelve months, then, 



Astronomical ClocK and Ecliptic 43 

the sun comes back again to the place in the 
sky which it occupied at the beginning of the 
year. Since the motion of the earth in its 
orbit is from west to east (the same as that 
of its rotation on its axis), it follows that 
the direction of the sun's apparent annual 
motion in the sky is from east to west 
(like its daily motion). Thus, while in 
fact the earth pursues a path, or orbit, round 
the sun, the sun seems to pursue a path 
round the earth. This apparent path of 
the sun, projected against the background 
of the sky, is called the ecliptic. The name 
arises from the fact that eclipses only occur 
when the moon is in or near the plane of the 
sun's apparent path. 

As the apparent motion of the sun round 
the ecliptic is caused by the real motion 
of the earth round the sun, we may regard 
the ecliptic as a circle marking the inter- 
section of the plane of the earth's orbit 
with the celestial sphere. In other words, 
if we were situated on the sun instead of on 
the earth, we would see the earth travelling 
round the sky in the circle of the ecliptic. 
We must keep this fact, that the ecliptic in- 
dicates the plane of the earth's orbit, firmly 
in mind, in order to understand what follows. 



44 THe Celestial SpHere 

The ecliptic is not coincident with the 
celestial equator, for the following reason: 
The axis of the earth's daily rotation is not 
parallel to, or does not point in the same 
direction as, the axis of its yearly revolution 
round the sun. As the axis of rotation is 
perpendicular to the equator, so the axis of 
the yearly revolution is perpendicular to 
the ecliptic, and since these two axes are 
inclined to one another, it results that the 
equator and the ecliptic must lie in different 
planes. The inclination of the plane of the 
ecliptic to that of the equator amounts to 
about 233/2 . 

As it is very important to have a clear 
conception of this subject, we may illustrate 
it in this way: Take a ball to represent the 
earth, and around it draw a circle to represent 
the equator. Then, through the centre of 
the ball, and at right angles to its equator, 
put a long pin to represent the axis. Set it 
afloat in a tub of water, weighting it so that 
it will be half submerged, and placing it in 
such a position that the pin will be not upright 
but inclined at a considerable angle from the 
vertical. Now, imagine that the sun is 
situated in the centre of the tub, and cause 
the ball to circle slowly round it, while 



Astronomical ClocK and Ecliptic 45 

maintaining the pin always in the same po- 
sition. Then the surface of the water will 
represent the plane of the ecliptic, or plane 
of the earth's orbit, and you will see that, 
in consequence of the inclination of the pin, 
the plane of the equator does not coincide 
with that of the ecliptic (or the surface of 
the water), but is tipped with regard to it 
in such a manner that one half of the equator 
is below and the other half above it. Instead 
of actually trying this experiment, it will be 
a useful exercise of the imagination to re- 
present it to the mind's eye just as if it 
were tried. 

We have said that the inclination of the 
equator to the ecliptic amounts to 23^°, 
and this angle should be memorised. Now, 
since both the ecliptic and the equator are 
great circles of the celestial sphere, i. e., 
circles whose planes cut through the centre 
of the sphere, they must intersect one 
another at two opposite points. In the 
experiment just described, these two points 
lie on opposite sides of the ball, where the 
equator cuts the level of the water. These 
points of intersection of the equator and the 
ecliptic on the celestial sphere are called 
the equinoxes, or equinoctial points, because 



46 THe Celestial SpHere 

when the sun appears at either of those points 
it is perpendicular over the equator, and when 
it is in that position day and night are of 
equal length all over the earth. (Equinox 
is from two Latin words meaning "equal 
night.") 

We shall have more to say about the 
equinoxes later, but for the present it is 
sufficient to remark that one of these points 
— that one where the sun is about the 21st 
of March, which is the beginning of astro- 
nomical spring — is the "Greenwich of the 
Sky," or the vernal equinox. The other, 
opposite, point is called the autumnal 
equinox, because the sun arrives there 
about the 23d of September, the beginning 
of astronomical autumn. The vernal equi- 
nox, as we have already seen, serves as a 
pointer on the dial of the sky. When it 
crosses the meridian of any place it is 
astronomical noon at that place. Its 
position in the sky is not marked by any 
particular star, but it is situated in the 
constellation Pisces, and lies exactly at the 
crossing point of the celestial equator and 
the ecliptic. The hour circle, running through 
this point, and through its opposite, the 
autumnal equinox, is the prime meridian of 




Saturn 

From a drawing by Trouvelot. 




Saturn 

Photographed at the Lick Observatory. 



Astronomical ClocK and Ecliptic 47 

the heavens, called the equinoctial colure. 
The hour circle at right angles to the equi- 
noctial colure, i. e., bearing to it the same 
relation that the prime vertical does to 
the meridian (see Sect. 4), is called the sol- 
stitial colure. This latter circle cuts 
the ecliptic at two opposite points, called 
the solstices, which lie half-way between the 
equinoxes. Since the ecliptic is inclined 
2 3/ / ^° to the plane of the equator, and since 
the solstices lie half-way between the two 
crossing points of the ecliptic and the equator, 
it is evident that the solstices must be 
situated 23^° from the equator, one above 
and the other below, or one north and the 
other south. The northern one is called 
the summer solstice, because the sun arrives 
there at the beginning of astronomical sum- 
mer, about the 22d of June, and the southern 
one is called the winter solstice, because the 
sun arrives there at the beginning of the 
astronomical winter, about the 22d of 
December. The name solstice comes from 
two Latin words meaning ' ' the standing still 
of the sun," because when it is at the solsti- 
tial points its apparent course through the 
sky is for several days nearly horizontal 
and its declination changes very slowly. 



48 THe Celestial SpHere 

Now, just as there are two opposite 
points in the sky at equal distances from the 
equator, which mark the poles of the ima- 
ginary axis about which the celestial sphere 
makes its diurnal revolution, so there are 
two opposite points at equal distances from 
the ecliptic which mark the poles of the 
imaginary axis about which the yearly 
revolution of the sun takes place. These 
are called the poles of the ecliptic, and they 
are situated 23^° from the celestial poles — 
a distance necessarily corresponding with 
the inclination of the ecliptic to the equator. 
The northern pole of the ecliptic is in the 
constellation Draco, which you may see any 
night circling round the North Star, together 
with the Great Dipper and Cassiopeia. 

9. Celestial Latitude and Longitude. We 
have seen that the celestial sphere is marked 
with imaginary circles resembling the circles 
of latitude and longitude on the earth, and 
that in both cases the circles are used for 
a similar purpose, viz., to determine the 
location of objects, in one case on the globe 
of the earth and in the other on the sphere 
of the heavens. It has also been explained 
that what corresponds to latitude on the 
celestial sphere is called declination, and 



Celestial Latitude and Longitude 49 

what corresponds to longitude is called 
right ascension. It happens, however, that 
these same terms, latitude and longitude, 
are also employed in astronomy. But, un- 
fortunately, they are based upon a differ- 
ent set of circles from that which has been 
described, and they do not correspond in 
the way that right ascension and declination 
do to terrestrial longitude and latitude. A 
few words must therefore be devoted to 
celestial latitude and longitude, as distin- 
guished from declination and right ascen- 
sion. 

Celestial latitude and longitude then, 
instead of being based upon the equator and 
the poles, are based upon the ecliptic and 
the poles of the ecliptic. Celestial latitude 
means distance north or south of the ecliptic 
(not of the equator), and celestial longitude 
means distance from the vernal equinox 
reckoned along the ecliptic (not along the 
equator). Celestial longitude runs, the 
same as right ascension, from west toward 
east, but it is reckoned in degrees instead 
of hours. Celestial latitude is measured 
the same as declination, but along circles 
running through the poles of the ecliptic 
instead of the celestial poles, and drawn 



50 THe Celestial SpKere 

perpendicular to the ecliptic instead of to 
the equator. Circles of celestial latitude 
are drawn parallel to the ecliptic and cen- 
tring round the poles of the ecliptic, and 
meridians of celestial longitude are drawn 
through the poles of the ecliptic and per- 
pendicular to the ecliptic itself. The 
meridian of celestial longitude that passes 
through the two equinoxes is the ecliptic 
prime meridian. This intersects the equi- 
noctial colure at the equinoctial points, 
making with it an angle of 23^°. The 
solstitial colure, which it will be remembered 
runs round the celestial sphere half-way 
between the equinoxes, is perpendicular 
to the ecliptic as well as to the equator, and 
so is common to the two systems of circles. 
It passes alike through the celestial poles 
and the poles of the ecliptic. It will also 
be observed that the vernal equinox is com- 
mon to the two systems of co-ordinates, be- 
cause it lies at one of the intersections of the 
ecliptic and the equator. In passing from one 
system to the other, the astronomer employs 
the methods of spherical trigonometry. 

10. The Zodiac and the Precession of the 
Equinoxes. The next thing with which we 
must make acquaintance is the zodiac. 



THe Zodiac and Equinoxes 51 




'Na 

Fig. 4. The Ecliptic and Celestial Latitude and Longitude. 

C, as in the other figures, is the place of the observer and 
Z is the zenith, but to avoid complication of details the 
circle of the horizon is not drawn, only the north-and- 
south line, N C S, being shown. 
Eq Eq' is the equator. 
Ec Ec' is the ecliptic. 
P and P' are the celestial poles, 
p and p' are the poles of the ecliptic. 
Na is the nadir. 

V is the vernal equinox, and A the autumnal equinox. 
The circle through s, parallel to the ecliptic, is a latitude 
circle. 

The circle p s p' is the ecliptic meridian of the star s. 
The circle P V P' A is the equinoctial colure. 
The circle p V p' A is the prime ecliptic meridian. 
The arc of the ecliptic meridian contained between the 
ecliptic and s measures the star's latitude. 
The arc of the ecliptic contained between V and the 
point where the ecliptic meridian psp' meets the 
ecliptic (or the angle V p s) measures the star's longi- 
tude east from V, the vernal equinox. 



52 THe Celestial Sphere 

We have learned that the ecliptic is a great 
circle of the celestial sphere inclined at an 
angle of 23^° to the equator, and crossing 
the latter at two opposite points called the 
equinoxes, and that the sun in its annual 
journey round the sky follows the circle of 
the ecliptic. Consequently, the place which 
the sun occupies at any time must be 
somewhere on the course of the ecliptic. 
The fact has been mentioned that as seen 
from the sun the earth would appear to 
travel round the ecliptic, whence the ecliptic 
may be regarded as the projection of the 
earth's orbit, or path, against the back- 
ground of the heavens. But, besides the 
earth there are seven other large planets, 
Mercury, Venus, Mars, Jupiter, Saturn, 
Uranus, and Neptune, which, like it, revolve 
round the sun, some nearer and some farther 
away. Now, the orbits of all of these planets 
lie in planes nearly coincident with that of 
the earth's orbit. None of them is inclined 
more than 7 from the ecliptic and most 
of them are inclined only one or two degrees. 
Consequently, as we watch these planets 
moving slowly round in their orbits we find 
that they are always quite close to the 
circle of the ecliptic. This fact shows that 



THe Zodiac and Equinoxes 53 

the solar system, i. e., the sun and its attend- 
ant planets, occupies a disk-shaped area 
in space, the outlines of which would be 
like those of a very thin round cheese, with 
the sun in the centre. The ecliptic indicates 
the median plane of this imaginary disk. 
The moon, too, travels nearly in this common 
plane, its orbit round the earth being inclined 
only a little more than 5° to the ecliptic. 

Even the early astronomers noticed these 
facts, and in ancient times they gave to the 
apparent road round the sky in which the 
sun and planets travel, in tracks relatively 
as close together as the parallel marks of 
v/heels on a highway, the name zodiac. 
They assigned to it a certain arbitrary width, 
sufficient to include the orbits of all the 
planets known to them. This width is 
8° on each side of the circle of the ecliptic, 
or 1 6° in all. They also divided the ring of 
the zodiac into twelve equal parts, corre- 
sponding with the number of months in a 
year, and each part was called a sign of the 
zodiac. Since there are 360 in a circle, 
each sign of the zodiac has a length of just 
30. ° To indicate the course of the zodiac 
to the eye, its inventors observed the con- 
stellations lying along it, assigning one 



54 THe Celestial SpKere 

constellation to each sign. Beginning at 
the vernal equinox, and running round east- 
ward, they gave to these zodiacal constel- 
lations, as well as to the corresponding signs, 
names drawn from fancy resemblances of 
the figures formed by the stars to men, 
animals, or other objects. The first sign 
and constellation were called Aries, the 
Ram, indicated by the symbol T ; the 
second, Taurus, the Bull, tf; the third, 
Gemini, the Twins, K ; the fourth, Cancer, 
the Crab, @ ; the fifth, Leo, the Lion, Q. ; the 
sixth, Virgo, the Virgin, TIP; the seventh, 
Libra, the Balance, ^= ; the eighth, Scorpio^ 
the Scorpion, tit; the ninth, Sagittarius, the 
Archer, tP ; the tenth, Capricornus, the 
Goat, vs ; the eleventh, Aquarius, the Water- 
Bearer, sw ; and the twelfth, Pisces, the 
Fishes, X. The name zodiac comes from a 
Greek word for animal, since most of the 
imaginary figures formed by the stars of 
the zodiacal constellations are those of 
animals. The signs and their corresponding 
constellations being supposed fixed in the 
sky, the planets, together with the sun and 
the moon, were observed to run through them 
in succession from west to east. 

When this system was invented, the signs 



THe Zodiac and Equinoxes 55 

and their constellations coincided in posi- 
tion, but in the course of time it was 
found that they were drifting apart, the signs, 
whose starting point remained the vernal 
equinox, backing westward through the sky 
until they became disjoined from their 
proper constellations. At present the sign 
Aries is found in the constellation next west 
of its original position, viz., Pisces, and so on 
round the entire circle. This motion, as 
already intimated, carries the equinoxes 
along with the signs, so that the vernal 
equinox, which was once at the beginning 
of the constellation Aries (as it still is at 
the beginning, or " first point," of the sign 
Aries), is now found in the constellation 
Pisces. 

To explain the shifting of the signs of the 
zodiac on the face of the sky we must 
consider the phenomenon known as the 
precession of the equinoxes, which is one of 
the most interesting things in astronomy. 
Let us refer again to the fact that the axis 
of the earth's daily rotation is inclined 23^° 
from a perpendicu ar to the plane of its 
yearly revolution round the sun, from which 
it results that the ecliptic is tipped at the 
same angle to the plane of the equator. 



56 THe Celestial SpKere 

Thus the sun, moving in the ecliptic, appears 
half the year above (or north of) the equator, 
and half the year below (or south of) -it, 
the crossing points being the two equinoxes. 
Now, this inclination of the earth's axis 
is the key to the explanation we are seeking. 
The direction in which the axis lies in space 
is a fixed direction, which can.be changed only 
by some outside force interfering. What 
we mean by this will become clearer if we 
think of the earth's axis as resembling the 
peg of a top, or the axis of a gyroscope. 
When a top is spinning smoothly, with its 
peg vertical, the peg will remain vertical as 
long as the spin is not diminished, and no 
outside force interferes. So, too, the axis of 
the spinning-wheel of a gyroscope keeps 
pointing in the same direction so persistently 
that the wheel is kept from falling. If it 
is so mounted that it is free to move in any 
direction, and if then you take the instru- 
ment in your hand and turn round with it, 
the axis will adjust itself in such a manner 
as to retain its original direction in space. 
This tendency of a rotating body to keep 
its axis of rotation fixed applies equally to 
the earth, whose axis, also, maintains a 
constant direction in space, except for a slow 



THe Zodiac and Hqviinoxes 57 

change produced by outside forces, which 
change constitutes the phenomenon of the 
precession of the equinoxes. 

We cannot too often recall the fact that 
the axis of the earth is coincident in direction 
with that of the celestial sphere, so that the 
earth's poles are situated directly under the 
celestial poles. But the poles of the ecliptic 
are 23}^° aside from the celestial poles. 
If the direction of the earth's axis and 
with it that of the celestial sphere, did 
not change at all, then the celestial poles 
and the poles of the ecliptic would always 
retain the same relative positions in the sky ; 
but, in fact, an exterior force, acting upon 
the earth, causes a gradual change in the 
direction of its axis, and in consequence of 
this change the celestial poles, whose position 
depends upon that of the earth's poles, 
have a slow motion of revolution about the 
poles of the ecliptic, in a circle of 23^° 
radius. The force which produces this effect 
is the attraction of the sun and the moon 
upon the protuberant part of the earth 
round its equator. If the earth were a 
perfect sphere, this force could not act, 
or would not exist, but since the earth is an 
oblate spheroid, slightly flattened at the 



58 THe Celestial SpKere 

poles, and bulged round the equator, the 
attraction acts upon the equatorial protuber- 
ance in such a way as to strive to pull the 
earth's axis into an upright position with 
respect to the plane of the ecliptic. But, 
in consequence of its spinning motion, the 
earth resists this pull, and tries, so to speak, 
to keep the inclination of its axis unchanged. 
The result is that the axis swings slowly round 
while maintaining nearly the same inclination 
to the plane of the ecliptic. 

Here, again, we may employ the illustration 
of a top. If the peg of the top is tipped a 
little aside, so that the attraction of gravi- 
tation would cause the top to fall flat on the 
table if it were not spinning, it will, as long 
as it continues to spin, swing round and round 
in a circle instead of falling. We cannot 
enter into a mathematical explanation of 
this phenomenon here, but the reader will 
find a clear popular account of the whole 
matter in Prof. John Perry's little book 
on Spinning Tops. It is sufficient here 
to say that the attraction of gravitation, 
tending to make the top fall, but really 
causing the peg to turn round and round, 
resembles, in its effect, the attraction of the 
sun and the moon upon the equatorial 




The Milky Way about Chi Cygni 

Photographed at the Lick Observatory by E. E. Barnard, with the six-inch 

Willard lens. 

Observe the cloud-like forms. 



THe Zodiac and Equinoxes 59 

protuberance of the earth, which makes the 
earth's axis turn round in space. 

Now, as we have said, this slow swinging 
round of the axis of the earth produces the 
so-called precession of the equinoxes. In a 
period of about 25,800 years, the axis makes 
one complete swing round, so that in that 
space of time the celestial poles describe 
a revolution about the poles of the ecliptic, 
which remain fixed. But since the equator 
is a circle situated half-way between the poles, 
it is evident that it must turn also. To 
illustrate this, take a round flat disk of tin, 
or pasteboard, to represent the equator and 
its plane, and perpendicularly through its 
centre run a straight rod to represent the 
axis. Put one end of the axis on the table, 
and, holding it at a fixed inclination, turn 
the upper end round in a circle. You 
will see that as the axis thus revolves, the 
disk revolves with it, and if you imagine a 
plane, parallel to the surface of the table, 
passing through the centre of the disk at 
the point where the rod pierces it, you will 
perceive that the two opposite points, where 
the edge of the disk intersects this imaginary 
plane, revolve with the disk. In one position 
of the axis, for instance, these points may lie 



60 The Celestial SpKere 

in the direction of the north and south sides 
of the room. When you have revolved the 
axis, and with it the disk, one quarter way 
round, the points will lie toward the east 
and west sides of the room. When you have 
produced a half revolution they will once 
more lie toward the north and south, but 
now the direction of the slope of the disk 
will be the reverse of that which it had at 
the beginning. Finally, when the revolution 
is completed, the two points will again lie 
north and south and the slope of the disk 
will be in the same direction as at the start. 
In this illustration the disk stands for the 
plane of the celestial equator, the rod for 
the axis of the celestial sphere, the imaginary 
plane parallel to the surface of the table for 
the plane of the ecliptic, and the two opposite 
points where this plane is intersected by the 
edge of the disk for the equinoxes. The 
motion of these points as the inclined disk 
revolves represents the precession of the 
equinoxes. This term means that the 
direction of the motion of the equinoxes, 
as they shift their place on the ecliptic, 
is such that they seem to precess, or move 
forward, as if to meet the sun in its annual 
journey round the ecliptic. The direction 



THe Zodiac and Equinoxes 61 

is from east to west, and thus the zodiacal 
signs are carried farther and farther westward 
from the constellations originally associated 
with them; for these signs, as we have said, 
are so arranged that they begin at the vernal 
equinox, and if the equinox moves, the 
whole system of signs must move with it. 
The amount of the motion is about 50". 2 
per year, and since there are 1,296,000" 
in a circle, simple division shows that the 
time required for one complete revolution of 
the equinoxes must be, as already stated in 
reference to the poles, about 25,800 years. 
A little over 2000 years ago the signs and the 
constellations were in accord; it follows, 
then, that about 23,800 years in the future, 
they will be in accord again. In the mean- 
while the signs will have backed entirely 
round the circle of the ecliptic. 

The attentive reader will perceive that 
the precession of the equinoxes, with its 
attendant revolution of the celestial poles 
round the poles of the ecliptic, must affect 
the position of the North Star. We have al- 
ready said that that star only happens to 
occupy its present commanding position 
in the sky. The star itself is motionless, 
or practically so, with regard to the earth, 



62 THe Celestial SpHere 

and it is the north pole that changes its place. 
At the present time the pole is about i° 10, 
from the North Star, in the direction of the 
Great Dipper, and it is slowly drawing nearer 
so that in about 200 years it will be less than 
half a degree from the star. After that the 
precessional motion will carry the pole in a 
circle departing farther and farther from the 
star, until the latter will have entirely lost 
its importance as a guide to the position of 
the pole. It happens, however, that several 
other conspicuous stars lie near this circle. 
One of these is Thuban, or Alpha Draconis 
(not now as bright as it once was), and this 
star at the time when it served as an indicator 
of the place of the pole, some 4600 years ago, 
was connected with a very romantic chapter 
in the history of astronomy. In the great 
pyramid of Cheops in Egypt, there is a long 
passage leading straight toward the north 
from a chamber cut deep in the rock under 
the centre of the pyramid, and the upward 
slope of this passage is such that it is believed 
to have been employed by the Egyptian 
astronomer-priests as a kind of telescope- 
tube for viewing the then pole star, and 
observing the times of its passage over the 
meridian — for even the North Star, since it 



THe Zodiac and Equinoxes 63 

is not exactly at the pole, revolves every 
twenty-four hours in a tiny circle about it, 
and consequently crosses the meridian twice 
a day, once above and once beneath the true 
pole. 

About 11,500 years in the future, the 
extremely brilliant star Vega, or Alpha 
Lyras, will serve as a pole star, although 
it will not be as near the pole as the North 
Star now is. At that time the North Star 
will be nearly 50 from the pole. In about 
21,000 years the pole will have come round 
again to the neighbourhood of Alpha 
Draconis, the star of the pyramid, and in 
about 25,800 years the North Star will have 
been restored to its present prestige as the 
apparent hub of the heavens. 

One curious irregularity in the motion 
of the earth's poles must be mentioned in 
connection with the precession of the equi- 
noxes. This is a kind of " nodding," known 
as nutation. It arises from variation in 
the effect of the attraction of the sun and 
the moon, due to the varying directions in 
which the attraction is exercised. As far 
as the sun is concerned, the precession is 
slower near the time of the equinoxes than 
in other parts of the year; in other words, 



64 THe Celestial SpKere 

it is most rapid in mid-summer and mid- 
winter when one or the other of the poles 
is turned sunward. A similar, but much 
larger, change takes place in the effect of the 
moon's attraction owing to the inclination 
of her orbit to the ecliptic. During about nine 
and a half years, or half the period of revolu- 
tion of her nodes (see Part III, Section 4), the 
moon tends to hasten the precession, and 
during the next nine and a half years to 
retard it. The general effect of the combi- 
nation of these irregularities is to cause the 
earth's poles to describe a slightly waving 
curve instead of a smooth circle round the 
poles of the ecliptic. There are about 
1400 of these "waves," or "nods," in the 
motion of the poles in the course of their 
26,000-year circuit. In accurate observation 
the astronomer is compelled to take account 
of the effects of nutation upon the apparent 
places of the stars. 

A very remarkable general consequence 
of the change in the direction of the earth's 
axis will be mentioned when we come to 
deal with the seasons. 




The Great Southern Star-Cluster w Centauri 

Photographed by S. I. Bailey at the South American Station of 
Harvard Observatory. 
Note the streaming of small stars around the cluster. The cluster itself 
is globular and its stars are too numerous to be counted, or even to be sepa- 
rately distinguished in the central part. 



PART II. 

THE EARTH. 



65 



PART II. 

THE EARTH. 

i. Nature, Shape, and Size of the Earth. 

The situation of the earth in the universe 
has been briefly described in Part I; it 
remains now to see what the earth is in itself, 
and what are some of the principal phenom- 
ena connected with it as a celestial body in- 
habited by observant and reasoning beings. 
We know by ordinary experience that the 
earth is composed of rock, sand, soil, etc., 
and generally covered, where there is no 
running or standing water in the form of 
rivers, lakes, or seas, with vegetation, such 
*as trees and grass. Further experience 
teaches us that the earth is very large, and 
that its surface is divided into wide areas 
of land and of water. The largest bodies of 
water, the oceans, taken all together, cover 
about 72 per cent., or nearly three-quarters 
of the entire surface of the earth. Investi- 
gations carried as far down as we can go 
show that the interior of the earth consists 
67 



68 TKe EartH 

of various kinds of rock, in which are con- 
tained many different kinds of metals. While 
there is reason for thinking that a high 
degree of temperature prevails deep in the 
earth, yet it appears evident, for other 
reasons, that, taken as a whole, it is solid 
and very rigid throughout. By methods, 
the history and description of which we have 
not here sufficient space to give, it has been 
proved that the earth is, in form, a globe, 
or more strictly an ellipsoid, slightly drawn 
in at the poles and swollen round the equator. 
The polar diameter is 7899 miles, and the 
equatorial diameter 7926 miles, the difference 
amounting to only 27 miles. Thus, for 
ordinary purposes, we may regard the earth 
as being a true sphere. The level of its 
surface, however, is varied by hills and 
mountains, which, though insignificant in 
comparison with the size of the whole earth, 
are enormous when compared with the 
structures of human hands. The loftiest 
known mountain on the earth, Mt. Everest 
in the Himalayas, has an elevation of 29,000 
feet above sea-level, and the deepest known 
depression of the ocean bottom, near the 
island of Guam in the Pacific, sinks 31,614 
feet below sea-level. Thus, the apex of the 



The Earth 69 

highest mountain is about eleven and a half 
miles in vertical elevation above the bottom 
of the deepest pit of the sea — a distance very 
considerably less than half the difference 
between the equatorial and polar diameters 
of the earth. 

It is believed that at the beginning of 
its history the earth was a molten mass, 
or perhaps a mass of hot gases and vapours 
like the sun, and that it assumed its present 
shape in obedience to mechanical laws, as 
it cooled off. The rotation caused it to 
swell round the equator and draw in at the 
poles. 

The outer part of the earth is called its 
crust, and geology shows that this has been 
subject to violent changes, such as upheavals 
and subsidences, and that in many places 
sea and land have interchanged places, 
probably more than once. Geology also 
shows that the rocks of the earth's crust 
are filled with the remains, or fossils, of 
plants and animals differing from those now 
existing, though related to them, and that 
many of these must have lived millions of 
years ago. Thus we see that the earth bears 
marks of an immense antiquity, and that 
it was probably inhabited during vast ages 



70 The Earth 

before the race of man had been developed. 
The origin of life upon the earth is unknown. 
2. The Attraction of Gravitation. Among 
the phenomena of life upon the earth, which 
are so familiar that only thoughtful persons 
see anything to wonder at in them, is what 
we call the "weight" of bodies. Every 
person feels that he is held down to the 
ground by his weight, and he knows that if 
he drops a heavy body it will fall straight 
toward the ground. But what is this weight 
which causes everything either to rest upon 
the earth or to fall back to it if lifted up and 
dropped? The answer to this question in- 
volves a principle, or "law," which affects 
the whole universe, and makes it what we 
see it. This principle is one of the great 
foundation stones of astronomy. It is 
called the law of gravitation, the word gravi- 
tation being derived from the Latin gravis, 
"heavy." Briefly stated, the law is that 
every body, or every particle of matter, at- 
tracts, or strives to draw to itself, every other 
body, or particle of matter. This force is called 
the attraction of gravitation. A large body 
possesses more attractive force than a 
small one, in proportion to the mass, or 
quantity of matter, that it contains. The 



THe Attraction of Gravitation 71 

earth, being extremely large, holds all bodies 
on its surface with a force proportionate 
to its great mass. This explains why we 
possess what we call weight, which is simply 
the effect of the attraction of the earth upon 
our bodies. A large body is heavier, or 
drawn with more force by the earth, than a 
small one (composed of the same kind of 
matter), because it has a greater mass. 
The body really attracts the earth as much 
as the earth attracts the body, but the 
amount of motion caused by the attraction 
is proportional to the respective masses of 
the attracting bodies, and since the mass of 
the earth is almost infinitely great in com- 
parison with that of any body that we can 
handle, the motion which the latter imparts 
to the earth is imperceptible, and it is the 
small body only that is seen to move under 
the force of the attraction. 

Now we are going to see how vastly 
important in its effects is the fact that the 
earth is spherical in form. Sir Isaac Newton, 
who first worked out mathematically the 
law of gravitation, proved that a spherical 
body attracts, and is attracted, as if its 
entire mass were concentrated in a point at 
its centre. From this it follows that the 



72 The EartK 

attraction of the earth is exercised just as if 
the whole attractive force emanated from 
a middle point, and, that being so, the effect 
of the attraction is to draw bodies from all 
sides toward the centre of the earth. This 
explains why people on the opposite side of 
the earth, or under our feet, as we say, 
experience the same attractive force, or 
have the same weight, that we do. All 
round the earth, no matter where they may 
be situated, objects are drawn toward the 
centre. If at any point on the earth you 
suspend a plumb-line, and then, going one 
quarter way round, suspend another plumb- 
line, each of the lines will be vertical at 
the place where it hangs, and yet, the 
directions of the two lines will be at right 
angles to one another, since both point toward 
the centre of the earth. 

Knowing the manner in which the earth 
attracts, we have the means of determining 
its entire mass, or, as it is sometimes called, 
the weight of the earth. The principle 
on which this is done is easily understood, 
Suppose, for instance, that a small ball of 
lead, of known weight, is brought near a 
large ball, and delicately suspended in such 
a way that, by microscopic observation, the 



TKe Attraction of Gravitation 73 

movement imparted by the attraction of 
the large ball can be measured. The force 
required to produce this movement can be 
compared with the force of the earth's 
attraction which produces the weight of 
the ball, and thus the ratio of the mass 
of the earth to that of the ball is deter- 
mined. The total mass of the earth has 
been found to be equivalent to a "weight" 
of about 6,500,000,000,000,000,000,000 tons. 
The mean density of the earth compared with 
that of water is found to be about 5^, that 
is to say, the earth weighs 5^ times as much 
as a globe of water of equal size. 

Newton did not stop with showing the 
manner of the earth's attraction upon bodies 
on or near its surface; he proved that the 
earth attracted the moon also, and thus 
retained it in its orbit. To understand this 
we must notice another fact concerning 
the manner in which gravitation acts. Its 
force varies with distance. Experiment 
followed by mathematical demonstration, 
has proved that the variation of the attrac- 
tion is inversely proportional to the square 
of the distance. This simply means that if 
the distance between the two bodies con- 
cerned is doubled, the force of attraction will 



74 The Earth 

be diminished four times, 4 being the square 
of 2 ; and that if the distance is halved, the 
force will be increased fourfold. Increase 
the distance three times, and the force 
diminishes nine times; diminish the distance 
three times, and the force increases nine 
times, because 9 is the square of 3, and, as 
we have said, the force varies inversely, 
or contrarily, to the change of distance. 
Knowing this, Newton computed what the 
force of the earth's attraction must be on 
the moon, and he found that it was just 
sufficient to keep the latter moving round 
and round the earth. But why does not 
the moon fall directly to the earth? Because 
the moon had originally another motion 
across the direction of the earth's attraction. 
How it got this motion is a question into 
which we cannot here enter, but, if it were 
not attracted by the earth (or by the sun), 
the moon would travel in a straight line 
through space, like a stone escaping from a 
sling. The force of the attraction is just 
sufficient to make the moon move in an 
orbital path about the earth. 

The same principle was extended by 
Newton to explain the motion of the earth 
around the sun. The force of the sun's 



TKe Attraction of Gravitation 75 

attraction, calculated in the same way, 
can be shown to be just sufficient to retain 




Fig. 5. How the Earth Controls the Moon. 

Let C be the centre of the earth and M that of the moon. 
Suppose the moon to be moving in a straight line at such 
a velocity that it will, if not interfered with, go to A in 
one day. Now suppose the attraction of the earth to 
act upon it. That attraction will draw it to M'. Again 
suppose that at M' the moon were suddenly released 
from the earth's attraction; it would then shoot straight 
ahead to B in the course of the next day. But, in fact, 
the earth's attraction acts continually, and in the second 
day the moon is drawn to M". In other words the 
moon is all the time falling away from the straight line 
that it would pursue but for the earth's attraction, and 
yet it does not get nearer the earth but simply travels 
in an endless curve round it. 



the earth in its orbit and prevent it from 



76 TKe Earth 

travelling away into space. And so with 
all the other planets which revolve round 
the sun. And this applies throughout the 
universe. There are certain so-called double, 
or binary, stars, which are so close together 
that their attraction upon one another causes 
them to revolve in orbits about their common 
centre. In truth, all the stars attract the 
earth and the sun, but the force of this 
attraction is so slight on account of their 
immense distance that we cannot observe 
its effects. The reader who wishes to pursue 
this subject of gravitational attraction should 
consult more extensive works, such as Prof. 
Young's General Astronomy, or Sir George 
Airy's Gravitation. 

3. The Tides. The tides in the ocean 
are a direct result of the attraction of gravi- 
tation. They also involve in an interesting 
way the principle that a spherical body, like 
the earth, attracts and is attracted as if its 
entire mass were concentrated at its centre. 
The cause of the tides is the difference in 
the attraction of the sun and moon upon the 
body of the earth as a rigid sphere, and upon 
the water of the oceans, as a fluid envelope 
whose particles, while not free to escape from 
the earth, are free to move, or slide, among 



Photograph of a Group of Sun-spots 

Similar groups are frequently seen during periods of sun-spot maximum. 



TKe Tides 77 

one another in obedience to varying forces. 
The difference of the force of attraction 
arises from the difference of distance. Since 
the moon, because of her relative nearness, 
is the chief agent in producing tides we shall, 
at first, consider her tidal influence alone. 
The diameter of the earth is, in round 
numbers, 8000 miles; therefore, its radius 
is 4000 miles. From this it follows that the 
centre of the earth is 4000 miles farther from 
the moon than that side of the earth which 
is toward her at any time, and 4000 miles 
nearer than the side which is away from her. 
Consequently, her attraction must be stronger 
upon the water of the ocean lying just under 
her than upon the centre of the earth, and it 
must also be stronger upon the centre of the 
earth than upon the water of the ocean 
lying upon the side which is farthest from 
her. The result of these differences in the 
force of the moon's attraction is that the 
water directly under her tends away from 
the centre of the earth, while, on the other 
hand, the earth, considered as a solid sphere, 
tends away from the water on the side op- 
posite to that where the moon is, and these 
combined tendencies cause the water to 
rise, with regard to its general level, in two 



78 The Earth 

protuberances, situated on opposite sides of 
the earth. These we call tides. 

Some persons, when this statement is 
made, inquire: "Why, then, does not the 
moon take the water entirely away from the 
earth?" The answer is, that the effect of 
the tidal force is simply to diminish very 
slightly the weight of the water, or its ten- 
dency towards the earth's centre, but not 
to destroy, or overmaster, the gravitational 
control of the earth. The water retains 
nearly all its weight, for the tidal force of 
the moon diminishes it less than one part 
in 8,000,000. Still, this slight diminution 
is sufficient to cause the water to swell a 
little above its general level, at the points 
where it feels the effect of the tidal force. 
On the other hand, around that part of the 
earth which is situated half-way between the 
two tides, or along a diameter at right angles 
to the direction of the moon, the latter's 
attraction increases the weight of the water, 
i. £., its tendency toward the earth's centre 
(see Fig. 6). Perhaps this can better be 
understood, if we imagine the earth to be 
entirely liquid. In that case the difference 
in the force of the moon's attraction with 
difference of distance would be manifested 



THe Tides 



n 



in varying degrees throughout the earth's 
whole frame, and the result would be to draw 




Fig. 6. The Tidal Force of the Moon. 

The solid earth is represented surrounded by a shell of 
water. The water on the side toward the moon is more 
attracted than the centre of the earth, C ; the water on the 
opposite side is less attracted. The lines of force from 
the moon to the parts of the water lying toward A and 
B are inclined to the direct line between the centres of 
the earth and the moon, and the forces acting along 
these lines tend to draw the water in the directions 
shown by the arrow points. These are resultants of the 
horizontal and vertical components of the moon's 
attraction at the corresponding points on the earth, and 
the force acting along them tends to increase the weight 
of the water wherever the lines are inclined more toward 
the centre of the earth than toward the moon. On the 
side opposite the moon the same effects are produced in 
reverse, because on that side the general tendency is to 
draw the earth away from the water. Consequently if 
the earth did not rotate, and if it were surrounded with a 
complete shell of water, the latter would be drawn into 
an ellipsoidal shape, with the highest points under and 
opposite to the moon, and the lowest at the extremities 
of the diameter lying at right angles to the direction of 
the moon. 



the watery globe out into an ellipsoidal 
figure, having its greatest diameter in the 



80 The Earth 

line of the moon's attraction, and its smallest 
diameter at right angles to that line. The 
proportions of the ellipsoid would be such 
that the forces would be in equilibrium. 

Owing to a variety of causes, such as the 
rotation of the earth on its axis, which carries 
the water rapidly round with it ; the inertia 
of the water, preventing it from instantly 
responding to the tidal force; the irregular 
shape of the oceans, interrupted on all sides 
by great areas of land; their varying depth* 
producing differences of friction, and so on, 
the tidal waves do not appear directly under, 
or directly opposite to, the moon, and the 
calculation of the course and height of the 
actual tides, at particular points on the earth, 
becomes one of the most difficult problems in 
astronomical physics. 

We now turn to consider the effects of the 
sun's tidal force in connection with that of 
the moon. This introduces further compli- 
cations. The solar tides are only about 
two-fifths as high as the lunar tides, but they 
suffice to produce notable effects when they 
are either combined with, or act in opposition 
to, the others. They are combined twice a 
month — once when the moon is between the 
earth and the sun, at the time of new moon; 



The Tides 8l 

and again when the moon is in opposition 
to the sun, at the time of full moon. In 
these two positions the attractions of the sun 
and the moon must, so to speak, act together, 
with the result that the tides produced by 
them blend into a single greater wave. This 
combination produces what are called spring 
tides, the highest of the month. When, on 
the other hand, the moon is in a position at 
right angles to the direction of the sun, 
which happens at the lunar phases named 
first and last quarters, the solar and the 
lunar tides have their crests 90 ° apart, 
and, in a sense, act against one another, 
and then we have the neap tides, which are 
the lowest of the month. 

Without entering into a demonstration, 
it may here be stated as a fact to be memor- 
ised, that the tidal force exerted by any 
celestial body varies inversely as the cube 
of the distance. This is the reason why the 
sun, although it exceeds the moon in mass 
more than 25,000,000 times, and is situated 
only about 400 times as far away from the 
earth, exercises comparatively so slight a 
tidal force on the water of the ocean. If the 
tidal force varied as the square of the dis- 
tance, like the ordinary effects of gravitation, 

6 



82 The Earth 

the tides produced by the sun would be more 
than 150 times as high as those produced 
by the moon, and would sweep New York, 
London, and all the seaports of the world 
to destruction. In that case it might be 
possible, by delicate observations, to detect 
a tidal effect produced upon the oceans of 
the earth by the planet Jupiter. 

4. The Atmosphere. The solid globe of 
the earth is enveloped in a mixture of gases, 
principally oxygen and nitrogen, which we 
call the air, or the atmosphere, and upon 
whose presence our life and most other 
forms of life depend. The atmosphere 
is retained by the attraction of the earth, 
and it rotates together with the earth. 
If this were not so — if the atmosphere 
stood fast while the earth continued to spin 
within it — a terrific wind would constantly 
blow from the east, having a velocity at the 
equator of more than a thousand miles an 
hour. 

Exactly how high the atmosphere extends 
we do not know — it may not have any 
definite limits — but we do know that its 
density rapidly diminishes with increase 
of height above the ground, so that above 
an elevation of a few miles it becomes so rare 



THe Atmosphere 83 

that it would not support human life. The 
phenomena of meteors, set afire by the fric- 
tion of their swift rush through the upper 
air, prove, however, that there is a perceptible 
atmosphere at an elevation of more than a 
hundred miles. 

From an astronomical point of view, the 
most important effect of the presence of the 
atmosphere is its power of refracting light. 
By refraction is meant the property possessed 
by every transparent medium of bending, 
under particular circumstances, the rays of 
light which enter it out of their original 
course. The science of physics teaches us 
that if a ray of light passes from any trans- 
parent medium into another which is denser, 
and if the path of this ray is not perpen- 
dicular to the surface of the second medium, 
it will be turned from its original course 
in such a way as to make it more nearly 
perpendicular. Thus, if a ray of light passes 
from air into water at a certain slope to the 
surface, it will, upon entering the water, 
be so changed in direction that the slope 
will become steeper. Only if it falls per- 
pendicularly upon the water will it continue 
on without change of direction. Conversely 
a ray passing from a denser into a rarer 



84 TKe EartK 

medium is bent away from a perpendicular 
to the surface of the first medium, or its 
slope becomes less. This explains why, if 
we put a coin in a bowl, with the eye in such 
a position that it cannot see the coin over 
the edge, and then fill the bowl with water, 
the coin seems to be lifted up into sight. 
Moreover, if any transparent medium in- 
creases in density with depth, the amount 
of refraction will increase as the ray goes 
deeper, and the direction of the ray will be 
changed from a straight line into a curve, 
tending to become more and more perpen- 
dicular. 

Now all this applies to the atmosphere. 
If a star is seen in the zenith, its light falls 
perpendicularly into the atmosphere and 
its course is not deviated, or in other words 
there is no refraction. But if the star is 
somewhere between the zenith and the 
horizon, its light falls slopingly into the 
atmosphere, and is subject to refraction, 
the amount of bending increasing with 
approach to the horizon. Observation shows 
that the refraction of the atmosphere, which 
is zero at the zenith, increases to about half 
a degree (and sometimes much more, depend- 
ing upon the state of the air), near the 



THe Atmosphere 



85 



horizon. It follows that a celestial object 
seen near the horizon will ordinarily appear 
about half a degree above its true place. 
Since the apparent diameters of the sun 
and the moon are about half a degree, when 



/ \ 




Refraction. 



Suppose an observer situated at O on the earth. The sun, 
at S, has sunk below the level of his horizon, O H, but 
since the sun sends out rays in all directions there will 
be some, such as S A B, which will strike the atmosphere 
at A, and the refraction, tending to make the ray more 
nearly perpendicular to the surface of the atmosphere, 
will, instead of allowing it to go on straight over the 
observer's head to B, bend it down along the dotted line 
A O, and the observer will see the sun as if it lay in the 
direction of the dotted line O A S', which places the sun 
apparently above the horizon. 



they are rising or setting they can be seen 
on the horizon before they have really risen 
above it, or after they have really sunk 
below it. Tables of refraction at various 
altitudes have been prepared, and they have 
to be consulted in all exact observations of 
the celestial bodies. 



86 THe Earth 

5. Dip of the Horizon. Another correc- 
tion which has to be applied in many 
observations depends upon the sphericity 
of the earth. We have described the rational 
horizon, and pointed out how it differs from 
the sensible horizon. We have also said 
that at sea the sensible horizon nearly 
accords with the rational horizon (see 
Part I, Sect. 3). But the accord is not 
complete, owing to what is called the dip 
of the horizon. In fact, the sea horizon lies 
below the rational horizon by an amount 
varying with the elevation of the eye above 
the surface. Geometry enables us to deter- 
mine just what the dip of the horizon must 
be for any given elevation of the eye. A 
rough and ready rule, which may serve for 
many purposes, is that the square root of 
the elevation of the eye in feet equals the 
dip of the horizon in minutes of arc, or of 
angular measure. The reader will readily 
see that the dip of the horizon is a necessary 
consequence of the rotundity of the earth. 
It is because of this that, as a ship recedes 
at sea her hull first disappears below the 
horizon, and then her lower sails, and finally 
her top-sails. The use of a telescope does 
not help the matter, because a telescope 



Dip of tKe Horizon 



87 



only sees straight, and cannot bend the 
line of sight over the rim of the horizon. 



3 

> 
5 


— ^ ^N^ HORIZONTAL PLANE 





Fig. 8. Dip of the Horizon. 

It is to be remembered that it is the sensible horizon 
which dips, and not the rational horizon. The sensible 
horizon of the observer at the elevation A dips below the 
horizontal plane and he sees round the curved surface 
as far as a; in other words his skyline is at a. The 
observer at the elevation B has a sensible horizon still 
more inclined and he sees as far as b. If the observation 
were made from an immense height the observer would 
see practically half round the earth just as we see half 
round the globe of the moon. 

Atmospheric refraction, however, enables us 
to see an object which would be hidden by 
the horizon if there were no air. In navi- 



88 The Earth 

gation, which, as a science, is an outgrowth 
of astronomy, these things have to be care- 
fully taken into account. 

6. Aberration. A few words must be said 
about the phenomenon known as aberration 
of light. This is an apparent displacement 
of a celestial object due to the motion of 
the earth in its orbit. It is customary to 
illustrate it by imagining oneself to be in a 
shower of rain, whose drops are falling 
vertically. In such a case, if a person 
stands fast the rain will descend perpen- 
dicularly upon his head, but if he advances 
rapidly in any direction he will feel the drops 
striking him in the face, because his own 
forward motion is compounded with the 
downward motion of the rain so that the 
latter seems to be descending slantingly 
toward him. The same thing happens with 
the light falling from the stars. As the 
earth advances in its orbit it seems to meet 
the light rays, and they appear to come from 
a direction ahead of the flying earth. The 
result is that, since we see a star in the 
direction from which its light seems to come, 
the star appears in advance of its real 
position, or of the position in which we 
would see it if the earth stood fast. The 



Polar Streamers of the Sun, Eclipse of 1889 




The Solar Corona at the Eclipse of 187 1 

From drawings. 



Aberration 89 

amount by which the position of a star 
is shifted by aberration depends upon the 
ratio of the earth's velocity to the velocity 
of light. In round numbers this ratio is as 
1 to 10,000. The motion of the earth being 
in a slightly eccentric ellipse, the stars 
describe corresponding, but very tiny, ellipses 
once every year upon the background of the 
sky. But the precise shape of the ellipse 
depends upon the position of the star on 
the celestial sphere. If it is near one of the 
poles of the ecliptic, it will describe an 
annual ellipse which will be almost a circle, 
its greater diameter being 41" of arc. If it 
is near the plane of the ecliptic, it will 
describe a very eccentric ellipse, but the 
greater diameter will always be 41 ", although 
the shorter diameter may be immeasurably 
small. The effects of aberration have to 
be allowed for in all careful astronomical 
observation either of the sun or the stars. 
This is done by reducing the apparent 
place of the object to the place it would have 
if it were seen at the centre of its annual 
ellipse. 

7. Time. Without astronomical obser- 
vations we could have no accurate knowledge 
of time. The basis of the measurement of 



90 The Earth 

time is furnished by the rotation of the earth 
on its axis. We divide the period which the 
earth occupies in making one complete 
turn into twenty -four equal parts, or hours. 
The ascertainment of this period, called a 
day, depends upon observations of the stars. 
Suppose we see a certain star exactly on the 
meridian at some moment; just twenty-four 
hours later that star will have gone entirely 
round the sky, and will again appear on the 
meridian. The revolving heavens constitute 
the great clock of clocks, by whose movements 
all other clocks are regulated. We know 
that it is not the heavens which revolve, 
but the earth which rotates, but for conven- 
ience we accept the appearance as a substitute 
for the fact. The rotation of the earth is 
so regular that no measurable variation has 
been found in two thousand years. We have 
reasons for thinking that there must be a 
very slow and gradual retardation, owing 
principally to the braking action of the tides, 
but it is so slight that we cannot detect 
it with any means at present within our 
command. 

In Part I it was shown how the passage 
across the meridian of the point in the sky 
called the vernal equinox serves to indicate 



Time 91 

the beginning of the astronomical "day," 
but the position of the vernal equinox itself 
has to be determined by observations on the 
stars. By means of a telescope, so mounted 
that it can only move up or down, round a 
horizontal axis, and with the axis pointing 
exactly east and west so that the up and down 
movements of the telescope tube follow the 
line of the meridian, the moment of passage 
across the meridian of a star at any altitude 
can be observed. Observations of this nature 
are continually made at all great government 
observatories, such as the observatory at 
Washington or that at Greenwich, and at 
many others, and by their means clocks and 
chronometers are corrected, and a standard 
of time is furnished to the whole world. 

There are, however, three different ways 
of reckoning time, or, as it is usually said, 
three kinds of time. One is sidereal time, 
which is indicated by the passage of stars 
across the meridian, and which measures the 
true period of the earth's rotation; another 
is apparent solar time, which is indicated by 
the passage of the sun across the meridian; 
and a third is mean solar time, which is 
indicated by a carefully regulated clock, 
whose errors are corrected by star ob- 



92 The Earth 

servations. This last kind of time is that 
which is universally used in ordinary life 
(the use of sidereal time being confined to 
astronomy), so it is necessary to explain 
what it is and how it differs from apparent 
solar time. 

In the first place, the reason why sidereal 
time is not universally and exclusively used 
is because, although it measures the true 
period of the earth's rotation by the apparent 
motion of the stars, it does not exactly accord 
with the apparent motion of the sun; and, 
naturally, the sun, since it is the source of 
light for the earth, and the cause of the dif- 
ference between day and night, is taken 
for all ordinary purposes, as the standard 
indicator of the progress of the hours. The 
fact that it is mid-day, or noon, at any place 
when the sun crosses the meridian of that 
place, is a fact of common knowledge, which 
cannot be ignored. On the other hand, the 
vernal equinox, which is the "noon mark" 
for sidereal time, is independent of the 
alternation of day and night, and may be 
on the meridian as well at midnight as at 
mid-day. Before clocks and watches were 
perfected, the moment of the sun's passage 
over the meridian was determined by means 



Time 93 

of a gnomon, which shows the instant of 
noon by the length of a shadow cast by an 
upright rod. Since the apparent course of 
the sun through the sky is a curve, rising 
from the eastern horizon, attaining its great- 



6 
J-—\ 




■e- 



Fig. q. Sidereal and Solar Time. 

C is the centre of the earth, and O the place of an 
observer on the earth's surface. 

Suppose the sun at A to be in conjunction with the star S. 
Then, at the end of twenty-four sidereal hours, when the 
earth has made one turn on its axis and the place O has 
again come into conjunction with the star, the sun, in 
consequence of its yearly motion in the ecliptic, will 
have advanced to B, and the earth will have to turn 
through the angle A C B before O will overtake the sun 
and complete a solar day; wherefore the solar day is 
longer than the sidereal. 

est elevation where it meets the meridian, 
and thence declining to the western horizon, 
it is evident that the length of the shadow 
must be least when the sun is on the meridian, 
or at its maximum altitude. The gnomon, 
or the sun-dial, gives us apparent solar time. 
But this differs from sidereal time because, 
as we saw in Part I, the sun, in consequence 
of the earth's motion round it, moves 



94 The Earth 

about one degree eastward every twenty- 
four hours, and, since one degree is equal to 
four minutes of time, the sun rises about four 
minutes later, with reference to the stars, 
every morning. Consequently it comes four 
minutes later to the meridian day after day. 
Or, to put it in another way, suppose that 
the sun and a certain star are upon the 
meridian at the same instant. The star is 
fixed in its place in the sky, but the sun is 
not fixed ; on the contrary it moves about one 
degree eastward (the same direction as that 
of the earth's rotation) in twenty -four hours. 
Then, when the rotation of the earth has 
brought the star back to the meridian at the 
end of twenty-four sidereal hours, the sun, 
in consequence of its motion, will still be one 
degree east of the meridian, and the earth 
must turn through the space of another 
degree, which will take four minutes, before 
it can have the sun again upon the meridian. 
The true distance moved by the sun in 
twenty-four hours is a little less than one 
degree, and the exact time required for the 
meridian to overtake it is 3 min. 56.555 sec. 
Thus, the sidereal day (period of 24 hours) 
is nearly four minutes shorter than the solar 
day. 



Time 95 

It would seem, then, that by taking the 
sun for a guide, and dividing the period 
between two of its successive passages over 
the meridian into twenty-four hours, we 
should have a perfect measure of time, 
without regard to the stars; in other words, 
that apparent solar time would be entirely 
satisfactory for ordinary use. But, unfor- 
tunately, the apparent eastward motion of 
the sun is not regular. It is sometimes 
greater than the average and sometimes less. 
This variation is due almost entirely: first, 
to the fact that its orbit not being a perfect 
circle the earth moves faster when it is 
near perihelion, and slower when it is near 
aphelion; and, second, to the effects of the 
inclination of the ecliptic to the equator. 
In consequence, another measure of solar 
time is used, called mean solar time, in which, 
by imagining a fictitious sun, moving with 
perfect regularity through the ecliptic, the 
discrepancies are avoided. All ordinary 
clocks are set to follow this fictitious, or 
mean, sun. The result is that clock time 
does not agree exactly with sun-dial time, 
or, what is the same thing, apparent solar 
time. The clock is ahead of the real sun 
at some times of the year, and behind it at 



96 THe Earth 

other times. This difference is called the 
equation of time. Four times in the year 
the equation is zero, i.e., there is no difference 
between the clock and the sun. These 
times are April 15, June 14, Sept. 1, and 
Dec. 24. At four other times of the year 
the difference is at a maximum, viz. Feb. 11, 
sun 14 min. 27 sec. behind clock; May 14, 
sun 3 min. 49 sec. ahead of clock; July 26, 
sun 6 min. 16 sec. behind clock; Nov. 2, 
sun 16 min. 18 sec. ahead of clock. These 
dates and differences vary very slightly 
from year to year. 

But, whatever measures of time we may 
use, it is observation of the stars that fur- 
nishes the means of correcting them. 

8. Day and Night. The period of twenty- 
four hours required for one turn of the earth 
on its axis is called a day, and in astronomical 
reckoning it is treated as an undivided whole, 
the hours being counted uninterruptedly 
from o to 24; but nature has divided the 
period into two very distinct portions, one 
characterised by the presence and the other 
by the absence of the sun. Popularly we 
speak of the sunlighted portion as day and 
of the other as night, and there are no two 
associated phenomena in nature more com- 




Morehouse's Comet, October 15, 1908 

Photographed at the Yerkes Observatory by E. E. Barnard with the ten- 
inch Bruce telescope. Exposure one hour and a half. 
Note the detached portions which appeared to separate from the head 
and retreat up the line of the tail at enormous velocity. 



Morehouse's Comet, November 15, 1908 

Photographed at the Yerkes Observatory by E. E. Barnard, with the ten. 
inch Bruce telescope. Exposure forty minutes. 



Day and NigKt 97 

pletely in contrast one to the other. The 
cause of the contrast between day and night 
must have been evident to the earliest human 
beings who were capable of any thought at 
all. They saw that day inevitably began 
whenever the sun rose above the horizon, 
and as inevitably ceased whenever it sank 
beneath it. In all literatures, imaginative 
writers have pictured the despair of primeval 
man when he first saw the sun disappear 
and night come on, and his joy when he first 
beheld the sun rise, bringing day back with 
it. Even his uninstructed mind could not 
have been in doubt about the causal con- 
nection of the sun with daylight. 

We now know that the cause of the alter- 
nate rising and setting of the sun, and of its 
apparent motion through the sky, is the 
rotation of the earth. Making in our minds 
a picture of the earth as a turning globe 
exposed to the sunbeams, we are able to see 
that one half of it. must necessarily be il- 
luminated, while the other half is in darkness. 
We also see that its rotation causes these 
two halves gradually to interchange places 
so that daylight progresses completely round 
the earth once in the course of twenty-four 
hours. If the earth were not surrounded 



98 The EartK 

by an atmosphere, exactly one half of it 
would lie in the sunlight and exactly one 
half in darkness, but the atmosphere causes 
the illuminated part slightly to exceed the 
unilluminated part. The reason for this is 
twofold : first, because the atmosphere, being 
transparent and extending to a considerable 
height above the solid globe, receives rays 
from the sun after the latter has sunk below 
the horizon, and these rays cause a faint il- 
lumination in the sky after the sun as viewed 
from the surface of the ground has disap- 
peared; and, second, because the air has the 
property of refracting the rays of light, 
in consequence of which the sun appears 
above the horizon both a little time before 
it has actually risen and a little time after 
it has actually set. The faint illumination 
at the beginning and the end of the day 
is called twilight. Its cause is the reflection 
of light from the air at a considerable ele- 
vation above the ground. Observation 
shows that evening twilight lasts until the 
sun has sunk about i8° below the west- 
ern horizon, while morning twilight begins 
when the sun is still i8° below the nearest 
horizon. The length of time occupied by 
twilight, or its duration, depends upon the 



Day and NigHt 99 

observer's place on the earth and increases 
with distance from the equator. The length 
of twilight at any particular place also varies 
with the seasons. 

It will probably have occurred to the reader 
that, since day and night are ceaselessly 
chasing each other round the globe, it must 
be necessary to choose some point of begin- 
ning, in order to keep the regular succession 
of the days of the week. The necessity for 
this is evident as soon as we reflect that 
what is sunrise at one place on the earth, 
is sunset for a place situated half-way round, 
on the other side. To understand this it 
will be better, perhaps, to consider the phe- 
nomena of noon at various places. It is 
noon at any place when the sun is on the 
meridian of that place. But we have seen 
that every place has its own meridian; 
consequently, since the sun cannot be on 
the meridian of more than one place at a 
time, each different place (reckoning east 
and west, for, of course, all places lying 
exactly north or south of one another have 
the same meridian), must have its own local 
noontime. Since the sun appears to move 
round the earth from east to west, it will 
arrive at the meridian of a place lying east 



ioo THe EartH 

of us sooner than at our meridian, and it 
will arrive at our meridian sooner than at 
that of a place lying west of us, Thus, when 
it is noon at Greenwich, it is about 7 o'clock 
A.M., or five hours before noon, at New 
York, because the angular distance westward 
round the earth's surface from Greenwich 
to New York is, in round numbers, 75 °, 
which corresponds with five hours of time, 
there being 15 to every hour. At the same 
moment it will be 5 o'clock p.m., or five 
hours after noon, at Cashmere, because 
Cashmere lies 75 ° east of Greenwich. That 
is to say, the sun crosses the meridian of 
Cashmere five hours before it reaches the 
meridian of Greenwich, and it crosses the 
meridian of Greenwich five hours before it 
reaches that of New York. At a place 
half-way round the circumference of the 
globe, i.e. 180 either east or west of 
Greenwich, it will 'be midnight at the same 
instant when it will be mid-day, or noon, 
at Greenwich. Now let us consider this 
for a moment. 

It is customary to change the name of 
the day at midnight. Thus at the stroke 
of midnight, anywhere, Sunday gives place 
to Monday. Suppose, then, that the day 



Day and NigKt 



IOI 



when we see the sun on the meridian at 
Greenwich happens to be Sunday. Sunday 




Fig. 10. The Change of Day. 

The arrows show the direction in which the earth turns 
(from west to east). It is always noon at the place 
which is directly under the sun. Call it Sunday noon 
at Greenwich, at the top of the circle; then it is 10 a.m. 
Sunday at a point 30 west and. 2 p.m. Sunday at a point 
30 east, and so on. Exactly opposite to the noon point 
it is midnight. By common consent we change the 
name of the day, and the date, at midnight; consequently 
it is Sunday midnight just east of the vertical line at the 
bottom of the circle and Monday morning just west of 
it. If we cross that line going westward we shall pass, 
directly from Sunday to Monday, and if we cross it 
going eastward we shall pass directly from Monday to 
Sunday. Since, by convention, this is a fixed line on 
the earth's surface, the same change will take place no 
matter what the hour of the day may be. 



will then be, so to speak, twelve hours old 
at Greenwich, because it began there at 



102 The EartK 

the preceding midnight. Sunday will be 
only seven hours old at New York, where 
it also began at the preceding midnight. 
In California, 45 °, or three hours, still farther 
west than New York, Sunday will be only 
four hours old, since the local time there 
is only four hours after midnight. Go on 
over the Pacific Ocean, until we arrive at 
a point 180 , or twelve hours, west of Green- 
wich. There, evidently, Sunday will just 
have been born, the preceding day, Saturday, 
having expired at the stroke of midnight. 
Now if we just step over that line of 180 
in what day shall we be? It cannot be 
Sunday, because Sunday has just begun on 
the line itself. It cannot be Saturday, 
because that would be counting backward. 
Evidently it can be no other than Monday. 
Let us examine this a little more closely. 
It is Sunday noon at Greenwich. We now 
go round the earth eastward instead of 
westward. At 90 °, or six hours, east of 
Greenwich, we find that it is 6 p.m. Sunclay 
and at 180 , or twelve hours, east of 
Greenwich we find that it is Sunday midnight, 
or in other words Monday morning. But 
the line of 180 east of Greenwich coincides 
with the line of 180 west of Greenwich, 



Day and Nig'Ht 103 

which we formerly approached from the 
opposite direction. So we see that we were 
right in concluding that in stepping over that 
line from the east to the west side, we were 
passing from Sunday into Monday. It is 
on that line that each day vanishes and its 
successor takes its place. It is the " date- 
line" for the whole earth, chosen by the 
common consent of every civilised nation, 
just as we have seen that the meridian of 
Greenwich is the common reference line for 
reckoning longitude. It lies entirely in the 
Pacific Ocean, hardly touching any island, 
and it was chosen for this very reason, 
because if it ran over inhabited lands, like 
Europe or America, it would cause endless 
confusion. Situated as it is, it causes no 
trouble except to sea captains, and very 
little to them. If a ship crosses the line 
going westward the captain jumps his log- 
book one day forward. If it is, for instance, 
Wednesday noon, east of the line he calls it 
Thursday noon, as soon as he has passed over. 
If he is going eastward he drops back a day 
on crossing the line, as from Thursday noon 
to Wednesday noon. The date-line theo- 
retically follows the 1 80th meridian, but, in 
fact, in order to avoid certain groups of 



104 THe EartK 

islands, it bends about a little, while keeping 
its general direction from north to south. 

9. The Seasons. We now recall again 
what was said in Part I, about the inclination 
of the ecliptic, or the apparent path of the 
sun in the heavens, to the equator. Because 
of this inclination, the sun appears half the 
year above the equator and the other half 
below it. When it is above the equator 
for people living in the northern hemisphere, 
it is below the equator for those living 
in the southern hemisphere, and vice versa. 
This is because observers on opposite sides 
of the plane of the equator look at it from 
opposite points of view. For the northern 
observer the celestial equator appears south 
of the zenith ; for the southern observer it 
appears north of the zenith, its distance 
from the zenith, in both cases, increasing 
with the observers distance from the equator 
of the earth. If he is on the earth's equator, 
the celestial equator passes directly through 
the zenith. For convenience we shall sup- 
pose the observer to be somewhere in the 
northern hemisphere. 

Let us begin with that time of the year 
when the sun arrives at the vernal equinox. 
This occurs about the 21st of March. The 




Head of the Great Comet of 1861 

From a drawing by Warren De La Rue. 




Halley's Comet, May 5, 1910 

Photographed at the Yerkes Observatory by E E. Barnard, with the ten- 
inch Bruce telescope. 
Tnis was shortly before the passage of the comet between the earth and 
the sun, when some think its tail was thrown over us. 



THe Seasons 105 

sun is then perpendicular over the equator, 
daylight extends, uninterrupted, from pole 
to pole, and day and night (neglecting the 
effects of twilight and refraction) are of 
equal length all over the earth. Everywhere 
there are about twelve hours of daylight 
and twelve hours of darkness. This is the 
beginning of the astronomical spring. As 
time goes on, the motion of the sun in the 
ecliptic carries it eastward from the vernal 
equinox, and, at the same time, owing to the 
inclination of the ecliptic, it rises gradually 
higher above the equator, increasing its 
northern declination slowly, day after day. 
Immediately the equality of day and night 
ceases, and in the northern hemisphere 
the day becomes gradually longer in duration 
than the night, while in the southern hemi- 
sphere it becomes shorter. Moreover, be- 
cause the sun is now north of the equator, 
daylight no longer extends from pole to pole 
on the earth, but the south pole is in continual 
darkness, while the north pole is illuminated. 
You can illustrate this, and explain to 
yourself why the relative length of day and 
night changes, and why the sun leaves one 
pole in darkness while rising higher over the 
other, by suspending a small terrestrial 



106 The EartK 

globe with its axis inclined about 23^° from 
the perpendicular, and passing a lamp 
around it in a horizontal plane. At two 
points only in its circuit will the lamp be 
directly over the equator of the globe. Call 
one of these points the vernal equinox. 
You will then see that, when the lamp is 
directly over this point, its light illuminates 
the globe from pole to pole, but when it has 
passed round so as to be at a point higher 
than the equator, its light no longer reaches 
the lower pole, although it passes over the 
upper one. 

Now, with the lamp thus elevated above 
the equator, set the globe in rotation about 
its axis. You will perceive that all points 
in the upper hemisphere are longer in light 
than in darkness, because the plane dividing 
the illuminated and the unilluminated halves 
of the globe is inclined to the globe's axis 
in such a way that it lies beyond the upper 
pole as seen from the direction of the lamp. 
Consequently, the upper half of the globe 
above the equator, as it goes round, has more 
of its surface illuminated than unilluminated, 
and, as it turns on its axis, any point in 
that upper half, moving round parallel to 
the equator, is longer in light than in dark- 



THe Seasons 



107 



VERNAL EQUINOX 








SUN 



WINTER 
SOLSTICE 





AUTUMN EQUINOX. 



Fig. 11. The Seasons. 

The earth is represented at four successive points in its 
orbit about the sun. Since the axis of the earth is 
virtually unchangeable in its direction in space (leaving 
out of account the slow effects of the precession of the 
equinoxes), it results that at one time of the year, the 
north pole is inclined toward the sun and at the opposite 
time of the year away from it. It attains its greatest 
inclination sunward at the summer solstice, then the line 
between day and night lies 23^° beyond the north pole, 
so that the whole area within the arctic circle is in per- 
petual daylight. The days are longer than the nights 
throughout the northern hemisphere, but the day becomes 
longer in proportion to the night as the arctic circle is ap- 
proached, and beyond that the sun is continually above 
the horizon. In the southern hemisphere exactly the re- 
verse occurs. When the earth has advanced to the autumn 
equinox, the axis is inclined neither toward nor away from 
the sun. The latter is then perpendicular over the equa- 
tor and day and night are of equal length all over the earth . 
When the earth reaches the winter solstice the north pole 
is inclined away from the sun, and now it is summer in the 
southern hemisphere. At the vernal equinox again there 
is no inclination of the axis either toward or away from 
the sun, and once more day and night are everywhere 
equal. A little study of this diagram will show why on 
the equator day and night are always of equal length. 



108 The EartH 

ness. You will also observe that the ratio 
of length of the light to the darkness is 
greater the nearer the point lies to the pole, 
and that when it is within a certain dis- 
tance of the pole, corresponding with the 
elevation of the lamp above the equator, 
it lies in continual light — in other words, 
within that distance from the pole night 
vanishes and daylight is unceasing. At the 
same time you will perceive that round the 
lower pole there is a similar space within 
which day has vanished and night is unceas- 
ing, and that in the whole of the lower 
hemisphere night is longer than day. Exactly 
on the equator, day and night are always of 
equal length. 

Endeavour to represent all this clearly to 
your imagination, before actually trying 
the experiment, or consulting a diagram. 
If you try the experiment you may, instead 
of setting the axis of the globe at a slant, 
place it upright, and then gradually raise 
and lower the lamp as it is carried round the 
globe, now above and now below the equator. 

We return to our description of the actual 
movements of the sun. As it rises higher 
from the equator, not only does the day 
increase in length relatively to the night, 



THe Seasons 109 

but the rays of sunlight descend more nearly 
perpendicular upon the northern hemisphere. 
The consequence is that their heating effect 
upon the ground and the atmosphere increases 
and the temperature rises until, when the 
sun reaches its greatest northern declination, 
about the 226. of June (when it is 23^° 
north of the equator), the astronomical 
summer begins. This point in the sun's 
course through the circle of the ecliptic 
is called the summer solstice (see Part I, 
Sect. 8). Having passed the solstice, the 
sun begins to decline again toward the equa- 
tor. For a short time the declination 
diminishes slowly because the course of the 
ecliptic close to the solstice is nearly parallel 
to the equator, and in the meantime the 
temperature in the northern hemisphere 
continues to increase, the amount of heat 
radiated away during the night being less 
than that received from the sun during the 
day. This condition continues for about six 
weeks, the greatest heats of summer falling 
at the end of July or the beginning of August, 
when the sun has already declined far toward 
the equator, and the nights have begun 
notably to lengthen. But the accumulation 
of heat during the earlier part of the summer 



no The Earth 

is sufficient to counterbalance the loss caused 
by the declension of the sun. 

About the 23d of September the sun 
again crosses the equator, this time at the 
autumnal equinox, the beginning of the 
astronomical autumn, and after that it 
sinks lower and lower (while appearing 
to rise in the southern hemisphere), until 
about the 226. of December, when it reaches 
its greatest southern declination, 23^°, at 
the winter solstice, which marks the begin- 
ning of the astronomical winter. It is 
hardly necessary to point out that the south- 
ern winter corresponds in time with the 
northern summer, and vice versa. From 
the winter solstice the sun turns northward 
once more, reaching the vernal equinox 
again on the 21st of March. 

Thus we see that we owe the succession 
of the seasons entirely to the inclination 
of the earth's axis out of a perpendicular to 
the plane of the ecliptic. If there were no 
such inclination there would be climate but 
no seasons. There would be no summer 
heat, except in the neighbourhood of the 
equator, while the middle latitudes would 
have a moderate temperature the year round. 
Owing to the effects of refraction, perpetual 



THe Seasons m 

day would prevail within a small region 
round each of the poles. The sun would be 
always perpendicular over the equator. 

Two things remain to be pointed out with 
regard to the effect of the sun's annual motion 
in the ecliptic. One of these is the circles 
called the tropics. These are drawn round 
the earth parallel to the equator and at a 
distance of 23^° from it, one in the northern 
and the other in the southern hemisphere. 
The northern one is called the tropic of 
Cancer, because its corresponding circle 
on the celestial sphere runs through the 
zodiacal sign Cancer, and the southern one 
is called the tropic of Capricorn for a similar 
reason. The tropics run through the two 
solstices, and mark the apparent daily track 
of the sun in the sky when it is at either its 
greatest northern or its greatest southern 
declination. The sun is then perpendicular 
over one or the other of the tropics. That 
part of the earth lying between the tropics 
is called the torrid zone, because the sun is 
always not far from perpendicular over it, 
and the heat is very great. 

The other thing to be mentioned is the 
polar circles. These are situated 23^° 
from each pole, just as the tropics are situated 



112 The Earth 

a similar distance on each side of the equator. 
The northern is called the arctic, and the 
southern the antarctic circle. Those parts 
of the earth which lie between the tropics 
and the polar circles are called respectively 
the northern and the southern temperate 
zone. The polar circles mark the limits of 
the region round each pole where the sun 
shines continuously when it is at one or the 
other of the solstices. If the reader will 
recall the experiment with the globe and the 
lamp, he will perceive that these circles 
correspond with the borders of the circular 
spaces at each pole of the globe which are 
alternately carried into and out of the full 
light as the lamp is elevated to its greatest 
height above the equator or depressed to its 
greatest distance below it. At each pole, 
in turn, there are six months of continual 
day followed by six months of continual 
night, and when the sun is at one of the 
solstices it just touches the horizon on the 
corresponding polar circle at the hour that 
marks midnight on the parts of the earth 
which lie outside the polar circles. This 
is the celebrated phenomenon of the "mid- 
night sun." At any point within the polar 
circle concerned, the sun, at the hour of 




The Six-Tailed Comet of 1744 

From a contemporary drawing. 



THe Seasons 113 

midnight approaches the horizon but does 
not touch it, its midnight elevation in- 
creasing with nearness to the pole, while 
exactly at the pole itself the sun simply moves 
round the sky once in twenty-four hours 
in a circle practically parallel to the horizon. 
It is by observations on the daily movement 
of the sun that an explorer seeking one of 
the earth's poles during the long polar day 
is able to determine when he has actually 
reached his goal. 

The reader will have remarked in these 
descriptions how frequently the angle of 
2 33 / 2° turns up, and he should remember that 
it is, in every case, due to the same cause, 
viz., the inclination of the earth's axis from 
a perpendicular to the ecliptic. 

A very remarkable fact must now be 
referred to. Although the angular distance 
that the sun has to travel in passing first from 
the vernal equinox to the autumnal equinox, 
on the northern side of the equator, and then 
back again from the autumnal equinox to 
the vernal equinox, on the southern side of 
the equator, is the same, the time that it 
occupies in making these two half stages 
in its annual journey is not the same. 
Beginning from the 21st of March and 



114 The EartH 

counting the number of days to the 23d 
of September, and then beginning from the 
23d of September and counting the number 
of days to the next 21st of March, you will 
find that in an ordinary year the first 
period is seven days longer than the second. 
In other words, the sun is a week longer 
above the equator than below it. The 
reason for this difference is found in the fact 
that the orbit of the earth about the sun is 
not a perfect circle, but is a slightly elongated 
ellipse, and the sun, instead of being situated 
in the centre, is situated in one of the two 
foci of the ellipse, 3,000,000 miles nearer to 
one end of it than to the other. Now this 
elliptical orbit of the earth is so situated 
that the earth is nearest to the focus occupied 
by the sun, or in perihelion, about December 
31st, only a few days after the winter solstice, 
and farthest from the sun, or in aphelion, 
about July 1st, only a few days after the 
summer solstice. Thus the earth is nearer 
the sun during the winter half of the year, 
when the sun appears south of the equator, 
than during the summer half of the year, 
when the sun appears north of the equator. 
Now the law of gravitation teaches that 
when the earth is nearer the sun it must move 



THe Seasons 115 

more rapidly in its orbit than when it is 
more distant, from which it follows that the 
time occupied by the sun in its apparent 
passage from the vernal equinox to the au- 
tumnal equinox is longer than that occupied 
in the passage back from the autumnal to 
the vernal equinox. 

But while the summer half of the year 
is longer than the winter half in the northern 
hemisphere, the reverse is the case in the 
southern hemisphere. There the winter is 
longer than the summer. Moreover, the 
winter of the southern hemisphere occurs 
when the earth is farthest from the sun, 
which accentuates the disadvantage. It has 
been thought that the greater quantity of 
ice about the south pole may be due to this 
increased length and severity of the southern 
winter. It is true that the southern summer, 
although shorter, is hotter than the northern, 
but while, theoretically, this should restore 
the balance as a whole, yet it would appear 
that the short hot summer does not, in fact, 
suffice to arrest the accumulation of ice. 

However, the present condition of things 
as between the two hemispheres will not 
continue, but in the course of time will be 
reversed. The reader will recall that the 



Ii6 The Earth 

precession of the equinoxes causes the axis 
of the earth to turn slowly round in space. 
At present the northern end of the earth's 
axis is inclined away from the aphelion and 
in the direction of the perihelion point of the 
orbit, so that the northern summer occurs 
when the earth is in the more distant part 
of its orbit,, and the winter when it is in the 
nearer part. But the precession swings the 
axis round westward from its present position 
at the rate of 5o".2 per year, while at the 
same time the position of the orbit itself 
is shifted (by the effects of the attraction 
of the planets) in such a manner that the 
aphelion and perihelion points, which are 
called the apsides, move round eastward 
at the rate of n^.25 per year. The combina- 
tion of the precession with the motion of 
the apsides produces a revolution at the rate 
of 6 1 "45 per year, which in the course of 
10,500 years will completely reverse the 
existing inclination of the axis with regard 
to the major diameter of the orbit, so that 
then the northern hemisphere will have its 
summer when the earth is near perihelion 
and its winter when it is near aphelion. 
The winter, then, will, for us, be long and 
severe and the summer short though hot. 



Year, Calendar, and MontH 117 

It has been thought possible that such a 
state of things may cause, in our hemisphere, 
a partial renewal of what is known in geology 
as a glacial period. A glacial period in the 
southern hemisphere would probably always 
be less severe than in the northern, because 
of the great preponderance of sea over land 
in the southern half of the globe. An ocean 
climate is more equable than a land climate. 
10. The Year, the Calendar, and the 
Month. A year is the period of time required 
for the earth to make one revolution in its 
orbit about the sun. But, as there are 
three kinds, or measures, of time, so there are 
three kinds, or measures, of the year. The 
first of these is called the sidereal year, but 
although, like sidereal time, it measures the 
true length of the period in question, it is 
not suitable for ordinary use. To understand 
what is meant by a sidereal year, imagine 
yourself to be looking at the earth from the 
sun, and suppose that at some instant you 
should see the earth exactly in conjunction 
with a star. When, having gone round the 
sun, it had come back again to conjunction 
with the same star, precisely one revolution 
would have been performed in its orbit, 
and the period elapsed would be a sidereal 



Ii8 The EartK 

year. Practically, the length of the sidereal 
year is determined by observing when the 
sun, in its apparent annual journey round 
the sky, has come back to conjunction with 
some given star. 

The second kind of year is called the tropi- 
cal year, and it is measured by the period 
taken by the sun to pass round the sky from 
one conjunction with the vernal equinox to 
the next. This period differs slightly from the 
first, because, owing to the precession of the 
equinoxes, the vernal equinox is slowly 
shifting westward, as if to meet the sun in its 
annual course, from which it results that the 
sun overtakes the equinox a little before 
it has completed a sidereal year. The 
tropical year is about twenty minutes shorter 
than the sidereal year. It is, however, 
more convenient for ordinary purposes, 
because we naturally refer the progress of 
the year to that of the seasons, and, as we 
have seen, the seasons depend upon the 
equinoxes. 

But yet the tropical year is not entirely 
satisfactory as a measure of time, because 
the number of days contained in it is not an 
even one. Its length is 366 days, 5 hours, 
48 minutes, 46 seconds. Accordingly, as 



Year, Calendar, and MontK 119 

the irregularities of apparent solar time were 
avoided by the invention of mean solar time, 
so the difficulty presented by the tropical 
year is gotten rid of, as far as possible, by 
means of what is called the civil year, or 
the calendar year, the average length of 
which is almost exactly equal to that of 
the tropical year. This brings us to the 
consideration of the calendar, which is as 
full of compromises as a political treaty — 
but there is no help for it since nature did 
not see fit to make the day an exact fraction 
of the year, or, in other words, to make the 
day and the year commensurable quantities 
of time. 

Without going into a history of the reforms 
that the calendar has undergone, which would 
demand a great deal of space, we may simply 
say that the basis of the calendar we use to- 
day was established by Julius Caesar, with 
the aid of the Greek astronomer Sosigenes. 
This is the Julian calendar, and the reformed 
shape in which it exists at present is called 
the Gregorian calendar. Caesar assumed 
365M days as the true length of the year, 
and, in order to get rid of the quarter day, 
ordered that it should be left out of account 
for three years out of every four. In the 



120 THe EartH 

fourth year the four quarter days were added 
together to make one additional day, 
which was added to that particular year. 
Thus the ordinary years were each 365 
days long and every fourth year was 366 
days long. This fourth year was called 
the bissextile year. It was identical with 
our leap year. The days of both the ordin- 
ary and the leap years were distributed among 
the twelve months very much as we distrib- 
ute them now. 

But Caesar's assumption of 365^ days 
as the length of the year was erroneous, 
being about umin. 14 sec. longer than the 
real tropical year. In the sixteenth century 
this error had accumulated to such a degree 
that the months were becoming seriously 
disjointed from the seasons with which they 
had been customarily associated. In con- 
sequence, Pope Gregory XIII, assisted by 
the astronomer Clavius, introduced a slight 
reform of the Julian calendar. The accumu- 
lated days were dropped, and a new start 
taken, and the rule for leap year was changed 
so as to read that ''all years, whose date- 
number is divisible by four without a remain- 
der are leap years, unless they are century 
years (such as 1800, 1900, etc.). The 



Year, Calendar, and MontH 121 

century years are not leap years, unless their 
date number is divisible by 400, in which 
case they are." And this is the rule as it 
prevails to-day, although there is now 
(19 1 2) serious talk of undertaking a new 
revision. But the Gregorian calendar is 
so nearly correct that more than 3000 years 
must elapse before the length of the year 
as determined by it will differ by one day 
from the true tropical year. 

The subject of the reform of the calendar 
is a very interesting one, but, together with 
that of the rules for determining the date of 
Easter, its discussion must be sought in more 
extensive works. 

There is one other measure of time, 
depending upon the motion of a heavenly 
body, which must be mentioned. This is 
the month, or the period required for the 
moon to make a revolution round the earth. 
Here we encounter again the same difficulty, 
for the month also is incommensurable with 
the year. Then, too, the length of the month 
varies according to the way in which it is 
reckoned. We have, first, a sidereal revo- 
lution of the moon, which is measured by the 
time taken to pass round the earth from one 
conjunction with a star to the next. This 



122 TKe Earth 

is, on the average, 27 days, 7 hours, 43 min- 
utes, 12 seconds. Next we have asynodical 
revolution of the moon, which is measured 
by the time it takes in passing from the phase 
of new moon round to the same phase again. 
This seems the most natural measure of a 
month, because the changing phases of the 
moon are its most conspicuous peculiarity. 
(These will be explained in Part III.) The 
length of the month, as thus measured, is, 
on the average, 29 days, P i2 hours, 44 minutes, 
3 seconds. The reason why the synodical 
month is so much longer than the sidereal 
month is because new moon can occur only 
when the moon is in conjunction with the sun, 
i.e. exactly between the earth and the sun, and 
in the interval between two new moons the 
sun moves onward, so that for the second con- 
junction the moon must go farther to overtake 
the sun. It will be observed that both of the 
month measures are given in average figures. 
This is because the moon's motion is not 
quite regular, owing partly to the eccentricity 
of its orbit and partly to the disturbing 
effects of the sun's attraction. The length of 
the sidereal revolution varies to the extent of 
three hours, and that of the synodical revo- 
lution to the extent of thirteen hours. 



Year, Calendar, and Month. 123 

But, whichever measure of the month we 
take, it is incommensurate with the year, 
i.e. there is not an even number of months 
in a year. In ancient times ceaseless efforts 
were made to adjust the months to the 
measure of the year, but we have practically 
given up the attempt, and in our calendar 
the lunar months shift along as they will, 
while the ordinary months are periods of a 
certain number of days, having no relation 
to the movements of the moon. 

It has been thought that the period called 
a week, which has been used from time 
immemorial, may have originated from the 
fact that the interval from new moon to the 
first quarter and from first quarter to full 
moon, etc., is very nearly seven days. But 
the week is as incorrigible as all its sisters 
in the discordant family of time, and there 
is no more difficult problem for human inge- 
nuity than that of inventing a system of 
reckoning, in which the days, the weeks, 
the months, and the years shall be adjusted 
to the closest possible harmony. 




Spiral Nebula in Ursa Major (M 101) 

Photographed at the Lick Observatory by J. E. Keeler, with the Crossley 
reflector. Exposure four hours. 
Note the appearance of swift revolution, as if the nebula were throwing 
itself to pieces like a spinning pin-wheel. 




The Whirlpool Nebula in Canes Venatici 

Photographed at the Lick Observatory by J. E. Keeler, with the Crossley 

reflector. Exposure four hours. 

Note the " beading " of the arms of the whirling nebula. 



PART III. 

THE SOLAR SYSTEM. 



125 



PART III. 

THE SOLAR SYSTEM. 

i. The Sun. By the term solar system 
is meant the sun together with the system 
of bodies (planets, asteroids, comets and 
meteors) revolving round it. The sun, being 
a star, every other star, for all that we can 
tell, may be the ruler of a similar system. 
In fact, we know that a few stars have huge 
dark bodies revolving round them, which 
may be likened to gigantic planets. The 
reason why the sun is the common centre 
round which the other members of the solar 
system move, is because it vastly exceeds 
all of them put together in mass, or quantity 
of matter, and the power of any body to set 
another body in motion by its attractive 
force depends upon mass. If a great body 
and a small body attract each other, both 
will move, but the motion of the small 
body will be so much more than that of the 
great one that the latter will seem, relatively, 
127 



128 THe Solar System 

to stand fast while the small one moves. 
Then, if the small body had originally a 
motion across the direction in which the 
great body attracts it, the result of the com- 
bination will be to cause the small body to 
revolve in an orbit (more or less elliptical 
according to the direction and velocity of 
its original motion) about the great body. 
If the difference of mass is very great, the 
large body will remain virtually immovable. 
This is the case with the sun and its planets. 
The sun has 332,000 times as much mass 
(or, we may say, is 332,000 times as heavy) 
as the earth. It has a little more than a 
thousand times as much mass as its largest 
planet, Jupiter. It has millions of times as 
much as the greatest comet. The conse- 
quence is that all of these bodies revolve 
around the sun, in orbits of various degrees 
of eccentricity, while the sun itself remains 
practically immovable, or just swaying a 
little this way and that, like a huntsman 
holding his dogs in leash. 

The distance of the sun from the earth — 
about 93,000,000 miles — has been determined 
by methods which will be briefly explained 
in the next section. Knowing its distance, 
it is easy to calculate its size, since the 



The Sun 129 

apparent diameter of all objects varies 
directly with their distance. The diameter 
of the sun is thus found to be about 866,400 
miles, or nearly no times that of the earth. 
In bulk it exceeds the earth about 1,300,000 
times, but its mass, or quantity of matter, 
is only 332,000 times the earth's, because 
its average density is but one quarter that of 
the earth. This arises from the fact that 
the earth is a solid, compact body, while the 
sun is a body composed of gases and vapours 
(though in a very compressed state). It 
is the high temperature of the sun which 
maintains it in this state. Its temperature 
has been calculated at about 16,000° Fahr- 
enheit, but various estimates differ rather 
widely. At any rate, it is so hot that the 
most refractory substances known to us 
would be reduced to the state of vapour, 
if removed to the sun. The quantity of heat 
received upon the earth from the sun can 
only be expressed in terms of the mechanical 
equivalent of heat. The unit of heat in 
engineering is the calorie, which means the 
amount of heat required to raise the tem- 
perature of one kilogram of water (2.2 pounds) 
one degree Centigrade (i°.8 Fahrenheit). 
Now observation shows that the sun fur- 



130 THe Solar System 

nishes 30 of these calories per minute upon 
every square metre (about 1.2 square yard) 
of the earth's surface. Perhaps there is 
no better illustration of what this means 
than Prof. Young's statement, that "the 
heat annually received on each square foot 
of the earth's surface, if employed in a perfect 
heat engine, would be able to hoist about a 
hundred tons to the height of a mile." Or 
take Prof. Todd's illustration of the me- 
chanical power of the sunbeams: "If we 
measure off a space five feet square, the 
energy of the sun's rays when falling verti- 
cally upon it is equivalent to one horse 
power." Astronomers ordinarily reckon the 
solar constant in "small calories," which are 
but the thousandth part of the engineer's 
calorie, and the latest results of the Smith- 
sonian Institution observations indicate that 
the solar constant is about 1.95 of these 
small calories per square centimeter per 
second. About 30 per cent, must be de- 
ducted for atmospheric absorption. 

Heat, like gravitation and like light, 
varies inversely in intensity with the square 
of the distance; hence, if the earth were 
twice as near as it is to the sun it would 
receive four times as much heat and four 



THe S\in 131 

times as much light, and if it were twice 
as far away it would receive only one quarter 
as much. This shows how important it is 
for a planet not to be too near, or too far 
from, the sun. The earth would be vapour - 
ised if it were carried within a quarter of 
a million miles of the sun. 

The sun rotates on an axis inclined about 
73^2° from a perpendicular to the plane 
of the ecliptic. The average period of its 
rotation is about 25 J days — we say " aver- 
age" because, not being a solid body, different 
parts of its surface turn at different rates. 
It rotates faster at the equator than at 
latitudes north and south of the equator, the 
velocity decreasing toward the poles. The 
period of rotation at the equator is about 
25 days, and at 40 ° north or south of the 
equator it is about 27 days. The direction 
of rotation is the same as that of the earth's. 

The surface of the sun, when viewed with 
a telescope, is often seen more or less spotted. 
The spots are black, or dusky, and frequently 
of very irregular shapes, although many of 
them are nearly circular. Generally they 
appear in groups drawn out in the direction 
of the solar rotation. Some of these groups 
cover areas of many millions of square miles, 



132 THe Solar System 

although the sun is so immense that even 
then they appear to the naked eye (guarded 
by a dark glass) only as small dark spots on 
its surface. The centres of sun-spots, are the 
darkest parts. Generally around the borders 
of the spots the surface seems to be more or 
less heaped up. Often, in large sun-spots, 
immense promontories, very brilliant, project 
over the dark interior, and many of these are 
prolonged into bridges of light, apparently 
traversing the chasms beneath. Constant 
changes of shape and arrangement take place, 
and there are few more astonishing telescopic 
objects than a great sun-spot. 

The spots are not always visible in equal 
numbers, and in some years but few are seen, 
and they are small. It has been found that 
they occur in periods, averaging about eleven 
years from maximum to minimum, although 
the length of the period is very irregular. 
It has also been observed that when the first 
spots of a new period appear, they are 
generally seen some 30 ° from the equator, 
either toward the north or toward the south, 
and that as the period progresses the spots 
increase in size, and seem to draw toward the 
equator, the last spots of the period being 
seen quite close to the equator, on one side 




" Tress Nebula" (N. G. C. 6992) in Cygnus 

Photographed at the Yerkes Observatory by G. W.Ritchey, with the two- 
foot reflector. 
Observe the strangely twisted look of this long curved nebula ; also the 
curious curves composed of minute stars near it. 



THe Svin 133 

or the other. The duration of individual 
spots is variable; some last but a day or 
two, and others continue for weeks, some- 
times being carried out of view by the rota- 
tion of the sun and brought into view again 
from the other side. 

The surface of the sun in the neighbour- 
hood of groups of spots is frequently marked 
by large areas covered with crinkled bright 
lines and patches, which are called faculae. 
These, which are the brightest parts of the 
sun, appear to be elevated above the general 
level. 

As to the cause and nature of sun-spots 
much remains to be learned. In 1908, Prof. 
George E. Hale, by means of an instrument 
called the spectro-heliograph, which selects 
out of the total radiation of the solar disk 
light peculiar to certain elements, and thus 
permits the use of that light alone in photo- 
graphing the sun, demonstrated that sun- 
spots probably arise from vortices, or 
whirling storms, and that these vortices 
produce strong magnetic fields in the sun- 
spots. The phenomenon may be regarded, 
says Prof. Hale, as somewhat analogous to 
a tornado or waterspout on the earth. The 
whirling trombe becomes wider at the top, 



134 THe Solar System 

carrying the gases from below upward. At 
the centre of the storm the rapid rotation 
produces an expansion which cools the 
gases and causes the appearance of a com- 
paratively dark cloud, which we see as the 
sun-spot. The vortices whirl in opposite 
directions on opposite sides of the sun's 
equator, thus obeying the same law that 
governs the rotation of cyclones on the 
earth. 

It has long been a question whether the 
condition of the sun as manifested by the 
spots upon its surface has an influence upon 
the meteorology of the earth. It is known 
that the sun-spot period coincides closely 
with periodical changes in the earth's mag- 
netism, and great outbursts on the sun have 
frequently been immediately followed by 
violent magnetic storms and brilliant displays 
of the aurora borealis on the earth. 

The sun undoubtedly exercises other in- 
fluences upon the earth than those familiar 
to us under the names of gravitation, light, 
and heat; but the nature of these other 
influences is not yet fully understood. 

The brilliant white surface of the sun is 
called the photosphere. It has been likened 
to a shell of intensely hot clouds, consisting 



The S\in 135 

of substances which are entirely vaporous 
within the body of the sun. Above the 
photosphere lies an envelope, estimated to 
be from 5000 to 10,000 miles thick, known 
as the chromosphere. It consists mainly 
of hydrogen and helium, and when seen 
during a total eclipse, when the globe of 
the sun is. concealed behind the moon, it pre- 
sents a brilliant scarlet colour. Above this 
are frequently seen splendid red flame-like 
objects, named prominences. They are of 
two varieties — one cloud-like in appearance, 
and the other resembling spikes, or trees 
with spreading tops, — but often their forms 
are infinitely varied. The latter, the so- 
called eruptive prominences, exhibit rapid 
motion away from the sun's surface, as if 
they consisted of matter which has been 
ejected by explosion. Occasionally these 
objects have been seen to grow to a height 
of several hundred thousand miles, with 
velocities of two or three hundred miles 
per second. 

The sun has still another envelope, of 
changing form, — the corona. This appar- 
ently consists of rare gaseous matter, whose 
characteristic constituent is an element un- 
known on the earth, called coronium. The 



136 TKe Solar System 

corona appears in the form of a luminous 
halo, surrounding the hidden sun during 
a total eclipse, and it often extends outward 
several million miles. . Its shape varies in 
accordance with the sun-spot period. It has 
a different appearance and outline at a time 
of maximum sun-spots from those which it 
presents at a minimum. There are many- 
things about the corona which suggest the 
play of electric and magnetic forces. The 
corona, although evidently always existing, 
is never seen except during the few minutes 
of complete obscuration of the sun that 
occurs in a total eclipse. This is because 
its light is not sufficiently intense to render 
it visible, when the atmosphere around the 
observer is illuminated by the direct rays 
of sunlight. 

2. Parallax. We now return to the 
question of the sun's distance from the earth, 
which we treat in a separate section, because 
thus it is possible to present, at a single 
view, the entire subject of the measurement 
of the distances of the heavenly bodies. 
The common basis of all such measurements 
is furnished by what is called parallax, which 
may be defined as the difference of direction 
of an object when viewed alternately from 



Parallax 137 

two separate points. The simplest example 
of parallax is found in looking at an object 
first with one eye and then with the other 
without, in the meantime, altering the 
position of the head. Suppose you sit in 
front of a window through which you can 
see the wall of a house on the opposite side 
of the street. Choose one of the vertical 
bars of the window-sash, and, closing the 
left eye, look at the bar with the right and 
note where it seems to be projected against 
the wall. Then close the right eye and open 
the left, and you will observe that the place 
of projection of the bar has shifted toward 
the right. This change of direction is due 
to parallax and its amount depends both 
upon the distance between the eyes and upon 
the distance of the window from the observer. 
To see how this principle is applied by the 
astronomer, let us suppose that we wish to 
ascertain the distance of the moon. The 
moon is so far away that the distance between 
the eyes is infinitesimal in comparison, 
so that no parallactic shift in its direction 
is apparent on viewing it alternately with 
the two eyes. But by making the obser- 
vations from widely separated points on 
the earth we can produce a parallactic 



138 Trie Solar System 

shifting of the moon's position which will 
be easily measurable. 

Let one of the points of observation be 
in the northern hemisphere and the other 
in the southern, thousands of miles apart. 
The two observers might then be compared 
to the eyes of an enormous head, each of 
which sees the moon in a measurably 
different direction. If the northern observer 
carefully ascertains the angular distance 
of the moon from his zenith, and the southern 
observer does the same with regard to his 
zenith, as indicated in Fig. 12, they can, 
by a combination of their measurements, 
construct a quadrilateral A c B M, of which 
all the angles may be ascertained from the 
two measurements, while the length of the 
sides A c and B c is already known, since 
they are each equal to the radius of the 
earth. With these data it is easy, by the 
rules of plane trigonometry, to calculate 
the length of the other sides, and also the 
length of the straight line from the centre 
of the earth to the moon. In all such 
cases the distance between the points of 
observation is called the base-line, whose 
length is known to start with, while the 
angles formed by the lines of direction at 



Parallax 139 

the opposite ends of the base-line are ascer- 
tained by measurement. 

In the case of the sun the distance con- 



Fig. 12. Parallax of the Moon. 

Let C be the centre of the earth, A and B the stations of 
-4wo observers, one in the northern, the other in the 
southern hemisphere, and M the moon. The lines C A Z 
and CBZ' indicate the direction of the zenith at A 
and B respectively. Subtracting the measured angles 
at A and B each from 180 gives the inside angles at 
those points. The angle at C is equal to the sum of the 
latitudes of A and B since they are on opposite sides of 
the equator. With three angles known, the fourth, at 
M, is found by simply subtracting their sum from 360 . 

cerned is so great (about 400 times that of 
the moon) that the parallax produced by 
viewing it from different points on the earth 
is too small to be certainly measured, and 



140 TKe Solar System 

a modification of the method has to be 
employed. One such modification, which 
has been much used, depends upon the fact 
that the planet Venus, being nearer the sun 
than the earth is, appears, at certain times, 
passing directly over the face of the sun. 
This is called a transit of Venus. During 
a transit, Venus is between three and four 
times nearer the earth than the sun is, and 
consequently its parallactic displacement, 
when viewed from widely separated points 
on the earth, is much greater than that of 
the sun. One of the ways in which the 
astronomer takes advantage of this fact is 
shown in Fig. 13. Let A and B be two 
points on opposite sides of the earth, but 
both somewhere near the equator. As Venus 
swings along in its orbit to pass between 
the earth and the sun, it will manifestly be 
seen just touching the sun's edge sooner 
from A than from B. The observer at A 
notes with extreme accuracy the exact 
moment when he sees Venus apparently 
touch the sun. Several minutes later, the 
observer at B will see the same phenomenon, 
and he also notes accurately the time of the 
apparent contact. Now, since we know from 
ordinary observation the time that Venus 




The Great Andromeda Nebula 

Photographed at the Yerkes Observatory by G. W. Ritchey, with the two- 
foot reflector. 
Observe the vast spiral, or elliptic, rings surrounding the central con- 
densation and the appearance of breaking up and re-shaping into smaller 
masses which some of the rings present. 



Parallax 



141 



requires to make one complete circuit of 
its orbit, we can, by simple proportion, 
calculate, from the time that it takes to pass 
from v to v 1 , the angular distance between 
the lines A S and B S, or in other words the 
size of the angle at S, which is equal to the 
parallactic displacement of the sun, as seen 
from opposite ends of the earth's diameter. 
Knowing, to begin with, the distance between 




Fig. 1 j . Parallax of the Sun from Transit of Venus. 
(For description see text.; 



A and B, we have the means of determining 
the length of all the other lines in the triangle, 
and hence the distance of the sun. This 
process is known as Delisle's method. 
There is another method, called Halley's, 
but in a brief treatise of this kind we cannot 
enter into a description of it. It suffices 
to say that both depend upon the same fun- 
damental principles. 

It must be added, however, that other 
ways of measuring the sun's distance than 



142 THe Solar System 

are afforded by transits of Venus have been 
developed. One of these depends upon 
observation of the asteroid Eros, which 
periodically approaches much nearer to the 
earth than Venus ever does. By observing 
the parallax of Eros, when it is nearest the 
earth, its distance can be ascertained, and 
that being known the distance of the sun 
is immediately deducible from it, because, 
by the third law of Kepler (to be explained 
later) , the relative distances of all the planets 
from the sun are proportional to their 
periods of revolution, so that if we know any 
one of the distances in miles w T e can calcu- 
late all the others. It is important here to 
state the angular amount of the sun's parallax, 
since it is a quantity which is continually 
referred to in books on astronomy. Accord- 
ing to the latest determination, based on 
observations of Eros, the solar parallax is 
8 ".807, which corresponds, in round num- 
bers, to a distance of 92,800,000 miles. A 
mean parallax of 8".796 is given by Mr. C. G. 
Abbot, based on a combination of results 
from a number of different methods, and this 
corresponds to a distance of 92,930,000 miles. 
To the astronomer, who seeks extreme ex- 
actness, the slightest difference is important. 



Parallax 143 

It should be noted that the figures 8". 807 or 
8 // .796 represent the parallactic displacement 
of the sun, as seen not from the opposite 
ends of the earth's entire diameter, but from 
opposite ends of its radius, or semi-diameter. 
Accordingly it is equal to half of the angle 
at S in Fig. 13. It is for convenience of 
calculation that, in such cases, the astronomer 
employs the semi-diameter, instead of the 
whole diameter for his base-line. 

The case of the stars must next be con- 
sidered, and now we find that the distances 
involved are so enormous that the diameter, 
or semi-diameter, of the earth is altogether 
too insignificant a quantity to afford an 
available base-line for the measurement. 
We should have remained forever ignorant 
of star distances but for the effects produced 
by the earth's change of place due to its 
annual revolution round the sun. The mean 
diameter of the earth's orbit is about 
186,000,000 miles, and we are able to make 
use of this immense distance as a base-line 
fpr ascertaining the parallax of a star. 
Suppose, for instance, that the direction 
of a star in the sky is observed on the 1st 
of January, and again on the 1st of July. 
In the meantime, the earth will have passed 



144 THe Solar System 

from one end of the base-line just described 
to the other, and unless the star observed 
is extremely remote, a careful comparison 
of the two measurements of direction will 
reveal a perceptible parallax, from which 
the actual distance of the star in question 
can be deduced. 

It is to be observed that if all the stars 
were equally distant this method would fail, 
because then there would be no " background " 
against which the shift of place could be 
observed ; all of the stars would shift together. 
But, in fact, the vast majority of the stars 
are so remote that even a base-line of 
186,000,000 miles is insufficient to produce 
a measurable shift in their direction. It is 
only the distances of the nearer stars which 
we can measure, and for them the multitude 
of more remote ones serves, like the wall of 
the house in the experiment with the window- 
bar, as a background on which the shift 
of place can be noted. Just as in calculations 
of the sun's parallax the semi-diameter of 
the earth is chosen for a base-line, so in the 
case of the stars the semi-diameter of the 
earth's orbit, amounting to 93,000,000 miles, 
forms the basis. Measured in this way the 
parallaxes of the nearest stars come out in 



Spectroscopic Analysis 145 

tenths, or hundredths, of a second of arc, 
or angular measurement. Thus the parallax 
of Alpha Centauri, the nearest known star ? 
is about o r/ .75, corresponding to a distance 
of about 26,000,000,000,000 miles. Now 
o' r .75 is a quantity inappreciable to the naked 
eye, and only to be measured with delicate 
instruments, and yet this almost invisible 
shift of direction is all that is produced by 
viewing the nearest star in the sky from the 
opposite ends of a base-line 93,000,000 miles 
long! 

3. Spectroscopic Analysis. We have next 
to deal with the constitution of the sun, 
or the nature of the substances of which it 
consists, and for this purpose we must first 
understand the operation of the spectroscope, 
in many respects the most wonderful in- 
strument that man has invented. It has 
given birth to the "chemistry of the sun" 
and the "chemistry of the stars," for by its 
aid we can be as certain of the nature of many 
of the substances of which they are made 
as we could be by actually visiting them. 

Fundamentally, spectroscopic analysis de- 
pends upon the principle of refraction, of 
which we have spoken in connection with 
the atmosphere. Although the most power- 



146 THe Solar System 

ful spectroscopes are now made on a different 
plan, the working of the instrument can 
best be comprehended by considering it in 
the form in which it was first invented, and 
in which it is still most often used. In its 
simplest form the spectroscope consists of 
a three-angle prism of glass, through which a 
ray of light is sent from the sun, star, or 
other luminous object to be examined. 
Glass, like air or water, has the property 
of refracting, or bending, all rays of light 
that enter it in an inclined direction. In 
passing through two of the opposite-sloping 
sides of a prism, the ray is twice bent, once 
on entering and again on leaving, in accord- 
ance with the principle that we have already 
mentioned (see Part II, Sect. 4). Still, 
merely bending the ray out of its original 
course would have no important result 
but for another associated phenomenon, 
known as dispersion. To explain dispersion 
we must recall the familiar fact that white 
light consists of a number of coloured com- 
ponents which, when united, make white. 
It is usual to speak of these primary, or 
prismatic, colours, as seven in number. 
These are red, orange, yellow, green, blue, 
indigo, and violet. Physicists now assign 



Spectroscopic .Analysis 147 




Fig. 14. Spectrum Analysis. 

The red is least turned by the prism from its original 
course and the violet most. If between the prism and 
the screen on which the spectrum falls there were inter- 
posed a gas of any kind that gas would absorb from the 
coloured rays passing through it the exact waves of light 
with which it would itself shine if it were made luminous 
by heat. It would not take out an entire section, or 
colour, from the spectrum, but only a small part of one 
or more of the colours, and the absence of these parts 
would be indicated on the screen by narrow black lines 
situated in various places; and these lines, in number and 
in situation, would differ with every different kind of 
gas that was interposed. If several kinds were inter- 
posed simultaneously they would all pick out their own 
peculiar rays from the light, and thus the spectrum 
would be crossed by a large number of dark lines, by the 
aid of which the nature of the various gases that pro- 
duced them could be told. The effect would be the 
same if the gases were interposed in the path of the 
white light before it enters the prism; — and this, in fact, 
is what happens when the spectrum of the sun, or a star, 
is examined — the absorption has already occurred at the 
surface of the luminous body before the light comes to 
the earth. 



148 TKe Solar System 

a different list of primary colours, but these, 
being generally familiar, will best serve our 
purpose. Without going into an explanation 
of the reasons, it will suffice to say that the 
waves of light producing these fundamental 
colours are not all equally affected by refrac- 
tion. The red is least, and the violet most, 
bent out of its course in passing through the 
prism, the other colours being bent more and 
more in proportion to their distance from the 
red. It follows that the ray, or beam, of 
light, which was white when it entered the 
prism, becomes divided or dispersed during 
its passage into a brush of seven different 
hues. Thus the prism may be said to ana- 
lyse the light into its fundamental colours, 
making them separately visible. This, as 
a scientific fact, dates from the time of New- 
ton. But Newton did not dream of the 
further magic that lay in the prism. 

It was noticed as early as 1801 that, when 
the light of the sun was dispersed in the way 
we have described, not only did the seven 
primary colours make their appearance, 
but across the ribbon-like band, called the 
spectrum, that was thus formed, ran a 
number of thin black lines, like narrow gaps 
in the band. The position of these lines was 



Spectroscopic Analysis 149 

carefully studied by a German astronomer, 
Fraunhofer, in 18 14, and they still bear the 
name of Fraunhofer lines. But the full 
explanation of them did not come until 1858 
when, with their aid, Kir schorl laid the founda- 
tions of spectrum analysis. 

This analysis is based upon the fact that 
the Fraunhofer lines are visual indications of 
the existence of certain substances in the 
sun. To explain this we must know three 
fundamental facts : 

1st: Every incandescent body that is either 
solid or liquid (or, if it consists of gases, 
is under high pressure) shines with compound 
white light, which, when dispersed by prisms, 
gives a continuous coloured band, or spectrum. 

2d: Every elementary substance when in 
the gaseous state, and under low pressure, 
if brought to incandescence by heat, shines 
with light which, when dispersed, gives a 
discontinuous spectrum, made up of separate 
bright lines; and each different element 
possesses its own peculiar spectral lines, 
never coinciding in position with the lines 
of any other element. 

3d: If the light from a body giving a 
continuous spectrum is caused to pass 
through a gas which is at a lower temperature, 



150 THe Solar System 

the gas will absorb precisely those light 
waves, of which its own spectrum is composed 
and will leave in the spectrum of the body 
a series of dark lines, or gaps, whose number 
and position indicate the nature of the gas 
whose absorptive action has produced them. 

Now, to apply these principles to the sun 
we have only to remember that it is a globe 
of gaseous substances, which are under 
great pressure, owing to the immense force 
of the sun's gravitation. Consequently it 
gives a continuous spectrum. But, at the 
same time, it is surrounded with gaseous 
envelopes, which are not as much com- 
pressed as the internal gases are, and which 
are at a lower temperature because they 
come in contact with the cold of surround- 
ing space. The light from the body of the 
sun must necessarily pass through these 
envelopes, and each of the gases of which 
they consist absorbs from the passing sun- 
light its own peculiar rays, with the result 
that the spectrum of the sun is seen crossed 
with a great number of black lines — the 
Fraunhofer lines. 

It will be remarked that the evidence 
which the Fraunhofer lines afford concerning 
the composition of the sun applies, strictly 



Spectroscopic Analysis 151 

only to the outer portion, or to the envelopes 
of gaseous matter that surround the interior 
globe. But since there is every reason to 
believe that the entire body of the sun is 
in a gaseous state, notwithstanding the 
internal pressure, and since we see that 
there is a continual circulation going on 
between the inner and outer portions, it is 
logical to conclude that essentially the same 
elements exist under varying conditions in 
all parts of the sun. 

In this way, then, we have learned the 
composition of the sun, and we find that it 
consists of virtually the same elementary 
substances found upon the earth, but exist- 
ing there in a gaseous or vaporous state. 
Among the elements which have been 
positively identified in the sun by means of 
their characteristic spectral lines are iron, 
calcium, sodium, aluminum, copper, zinc, 
silver, lead, potassium, nickel, tin, silicon, 
manganese, magnesium, cobalt, hydrogen, 
and at least twenty others which are likewise 
found upon the earth. Some elementary 
substances known on the earth, such as 
gold and oxygen, have not yet been certainly 
found in the sun, but there is every reason 
to believe that they all exist there, though 



15 2 THe Solar System 

perhaps under conditions which render their 
detection difficult or impossible. Helium 
was recognised as an element in the sun, 
by giving spectral lines different from any 
known substance, and it received its name 
"sun-metal," long before it was discovered 
on the earth. We have seen that there is 
at least one element in the sun, coronium, 
which, as far as we know, does not exist 
at all upon the earth, and it is not improbable 
that there may be others which have no 
counterparts on the earth. 

The same kind of analysis applies to the 
stars, no matter how far away they may be, 
so long as they give sufficient light to form 
a spectrum. And in this way it has been 
found that the stars differ somewhat from 
the sun and from one another in their com- 
position, and thus a classification of the stars 
has been made, and it has been possible to 
draw conclusions concerning their relative 
age, which show that some stars are com- 
paratively younger than the sun, others 
older, and others so far advanced in age, 
or evolution, that they are drawing near 
extinction. Many dark bodies also exist 
among the stars, which appear to be com- 
pletely extinguished suns. It only remains 



The Moon 153 

to add on this subject that, according to 
prevailing theories, the earth itself was once 
an incandescent body, shining with its own 
light, and at that time it, too, would have 
yielded a spectrum showing of what sub- 
stances it consisted. 

4. The Moon. The earth is a satellite 
of the sun, and the moon is a satellite of the 
earth. The mean, or average, distance of 
the sun from the earth is about 93,000,000 
miles; the mean distance of the moon is a 
little less than 239,000 miles. This distance 
is variable to the extent of about 31,000 
miles, owing to the eccentricity of the moon's 
orbit about the earth. That is, the moon is 
sometimes nearly 253,000 miles away, and 
sometimes only about 221, 600 . The diameter 
of the moon is 2163 miles. Its bulk is one- 
forty-ninth that of the earth, but its mass is 
only one-eightieth, because its mean density 
is only about six-tenths as great as the earth's. 

The moon appears to travel in an orbit 
round the earth, but in fact the orbit is 
always concave toward the sun, and the 
disturbing attraction of the earth, as the 
two move together round the sun, causes 
it to appear now on one side and now on the 
other. But we may treat the moon's 



154 THe Solar System 

orbit as if the earth were the true centre 
of force, the attraction of the sun being re- 
garded as the disturbing element. 

According to a mathematical theory, which 
has been largely accepted as probably true, 
but into which we cannot enter here (see 
Prof. George Darwin's The Tides, or Prof. 
R. Ball's Time and Tide), the moon was 
thrown off from the earth many ages ago, 
as a consequence of tidal "friction." As it 
moves round in its orbit the moon keeps 
the same face toward the earth. This fact is 
also ascribed to tidal influence. 

Apparently the moon has no atmosphere, 
or if it has any it is too rare to be certainly 
detected. On its surface, there is no appear- 
ance of water. Consequently we cannot 
suppose it to be inhabited, at least by any 
forms of life familiar to us on the earth. 
But when the moon is viewed with a telescope 
large relatively flat areas are seen, which 
some think may have been the beds of seas 
in ancient times. They are still called maria, 
or "seas," and are visible to the naked eye 
in the form of great irregular dusky regions. 
Nearly two-thirds of the surface of the moon, 
as we see it, consists of bright regions, which 
are very broken and mountainous. Most 




Spiral Nebula in Cepheus (H IV 76) 

Photographed at the Lick Observatory by J. E. Keeler, with the Crossley 

reflector. Exposure four hours. 

Observe that the central portion is only of stellar magnitude. 




Nebulous Groundwork in Taurus 

Photographed at the Yerkes Observatory by E. E. Barnard with io-ineh 
Bruce telescope. Exposure six hours twenty-eight minutes. 
Prof. Barnard has suggested that some of these dark lanes in rich regions 
nf stars are non-luminous nebulae. 



The Moon 155 

of the mountains of the moon are roughly 
circular, surrounding enormous depressions, 
which look like gigantic pits. For this 
reason they are called lunar volcanoes, but, 
to say nothing of their immense size — for 
many are fifty or sixty miles across — they 
differ in many ways from the volcanoes of 
the earth. It suffices to point out that what 
resemble volcanic craters are not situated, 
as is the case on the earth, at the summits 
of mountains, but are vast sink-holes, de- 
scending thousands of feet below the general 
surface of the moon. Their real origin is 
unknown, but it is possible that volcanic 
forces may have produced them. (For a 
description, with photographs, of these 
gigantic formations in the lunar world, see 
the present author's The Moon.) In ad- 
dition to the circular mountains, or craters, 
there are several long and lofty chains of 
lunar mountains much resembling terrestrial 
mountain ranges. 

As to the absence of air and water from 
the moon, some have supposed that they 
once existed, but, in the course of ages, 
have disappeared, either by absorption, 
partly mechanical and partly chemical, into 
the interior rocks, or by escaping into space 



156 THe Solar System 

on account of the slight force of gravity on 
the moon, which appears to be insufficient to 
enable it to retain, permanently, such volatile 
gases as oxygen, hydrogen, and nitrogen. 
This leads us to consider the force of the 
moon's attraction at its surface. We have 
seen that spherical bodies attract as if their 
whole mass were collected at their centres. 
We also know that the force of attraction 
varies directly as the mass of the attracting 
body and inversely as the square of the 
distance from its centre. Now the mass of 
the moon is one-eightieth that of the earth, so 
that, upon bodies situated at an equal dis- 
tance from the centres of both, the moon's 
attraction would be only one-eightieth of 
the earth's. But the diameter of the moon 
is not very much more than one quarter 
that of the earth, and for the sake of round 
numbers let us call it one quarter. It 
follows that an object on the surface of the 
moon is four times nearer the centre of 
attraction than is an object on the surface 
of the earth, and since the force varies 
inversely as the square of the distance the 
moon's attraction upon bodies on its surface 
is relatively sixteen times as great as the 
earth's. But the total force of the earth's 



TKe Moon 157 

attraction is eighty times greater than the 
moon's. In order, then, to find the real 
relative attraction of the moon at its surface 
we must divide 80 by 16, the quotient, 5, 
showing the ratio of the earth's force of 
attraction at its surface to that of the moon 
at its surface. In other words, this calcu- 
lation shows that the moon draws bodies 
on its surface with only one-fifth the force 
with which the same bodies would be drawn 
on the earth's surface. The weight of bodies 
of equal mass would, therefore, be only 
one-fifth as great on the moon as on the 
earth. 

But the real difference is greater than this, 
for we have used round numbers, which 
exaggerated the size of the earth as compared 
with that of the moon. If we employ the 
fractional numbers which show the actual 
ratio of the moon's radius (half-diameter) to 
that of the earth, we shall find that the 
weight of the same body would be only 
about one-sixth as great on the moon as 
on the earth. It has been thought that this 
relative lack of weight on the moon may 
account for the gigantic proportions assumed 
by its craters, since the same ejective force 
would throw volcanic matter to a much 



158 TKe Solar System 

greater height and distance there than on 
our planet. 

The connection of the slight force of gravity 
on the moon with its ability to retain an 
atmosphere is shown by the following con- 
siderations. It is possible to calculate for 
any planet of known mass the velocity with 
which a particle would have to move in 
order to escape from the control of that 
planet. In the case of the earth this critical 
velocity, as it is called, amounts to about 
7 miles per second, and in the case of the 
moon to only ij^ miles per second. Now 
the kinetic theory of gases informs us that 
their molecules are continually flying in 
all directions with velocities varying with 
the nature and the temperature of the gas. 
The maximum velocity of the molecules of 
oxygen is 1.8 miles per second, of hydrogen 
7.4 miles, of nitrogen 2 miles, of water 
vapour 2.5 miles. It is evident, then, that 
the force of the earth's attraction is sufficient 
permanently to retain all these gases except 
hydrogen, and in fact there is no gaseous 
hydrogen in the atmosphere, that element 
being found on the earth only in combination 
with other substances. But oxygen and 
nitrogen, which constitute the bulk of the 



The Moon 159 

atmosphere, have maximum molecular ve- 
locities much less than the critical velocity- 
above described. In the case of the moon, 
however, the critical velocity is less than those 
of the molecules of oxygen, nitrogen, and 
water vapour, to say nothing of hydrogen; 
therefore the moon cannot permanently 
retain them. We say ''permanently, " be- 
cause they might be retained for a time 
for the reason that the molecules of a gas 
fly in all directions, and continual collisions 
occur among them in the interior of the 
gaseous mass, so that it would be only those 
at the exterior of the atmosphere which 
would escape ; but gradually all that remained 
free from combination would get away. 

As the moon travels round the earth 
it shows itself in different forms, gradually 
changing from one into another, which are 
known as phases. If the moon shone with 
light of its own its outline would always be 
circular, like the sun's. The apparent change 
of form is due, first, to its being an opaque 
globe, reflecting the sunlight that falls upon 
it, and necessarily illuminated on only one 
side at a time; and second, to the fact that as 
it travels round the earth the half illuminated 
by the sun is sometimes turned directly 



i6o 



The Solar System 



toward us, at other times only partly toward 
us, and at still other times directly away 



LAST QUARTER 



TO me Sun 



NEW MOON 



f EARTH j 



FULL MOONI 



rO TM£ SU/V 



/ST QUARTER 



Fig. 15. The Phases of the Moon. 

As the moon goes round the earth in the direction indi- 
cated by the arrows, the sun remaining always on the 
left-hand side, it is evident that the illuminated half of 
the moon will be turned away from the earth at new 
moon, and toward it at full moon, while between these 
positions more or less will be seen according to the 
direction of the moon with regard to the sun. 

from us. When it is in that part of its orbit 
which passes between the sun and the earth, 



The Moon 161 

the moon, so to speak, has its back turned 
to us, the illuminated side being, of course, 
toward the sun. "It is then invisible, and 
this unseen phase is the true "new moon." 
It is customary, however, to give the name 
new moon to the narrow, sickle-shaped 
figure, which it shows in the west, after 
sunset, a few days after the date of the real 
new moon. The sickle gradually enlarges 
into a half circle as the moon passes away 
from the sun, and the half circle phase, 
which occurs when the moon arrives at a 
position in the sky at right angles to the 
direction of the sun, is called first quarter. 
After first quarter the moon begins to move 
round behind the earth, with respect to the 
sun, and when it has arrived just behind 
the earth, its whole illuminated face is 
turned toward the earth, because the sun, 
which causes the illumination, is on that 
same side. This phase is called full 
moon. Afterward the moon returns round 
the other part of its orbit toward its origi- 
nal position between the earth and the 
sun, and as it does so, it again assumes, 
first, the form of a half circle, which in this 
case is called third, or last, quarter, then 
that of a sickle, known as "old moon," and 



162 THe Solar System 

finally disappears once more to become new 
moon again. 

A perfectly evident explanation of these 
changes of form, clearer than any description, 
can be graphically obtained in this way. 
Take a billiard ball, a croquet ball, or a 
perfectly round, smooth, and tightly rolled 
ball of white yarn, and, placing yourself 
not too near a brightly burning lamp and 
sitting on a piano stool (in order to turn 
more easily), hold the ball up in the light, 
and cause it to revolve round you by turning 
upon the stool. As it passes from a position 
between you and the lamp to one on the oppo- 
site side from the lamp, and so on round 
to its original position, you will see its 
illuminated half go through all the changes 
of form exhibited by the moon, and you will 
need no further explanation of the lunar 
phases. 

The Harvest Moon and the Hunter's 
Moon, which are popularly celebrated not 
only on account of their romantic associa- 
tions, but also because in some parts of the 
world they afford a useful prolongation of 
light after sunset, occur only near the time 
of the autumnal equinox, and they are 
always full moons. The full moon nearest 




Nebula in Sagittarius (M 8) 

Photographed at the Lick Observatory by J. E. Keeler, with the Crossley 
reflector. Exposure three hours. 
Note the clustering of stars over the whole field, the intricate forms of 
the nebula, and particularly the curious black spots, or " holes," resembling 
drops of ink. 



ILclipses 163 

the date of the equinox, September 23d, is 
the Harvest Moon, and the full moon next 
following is the Hunter's Moon. Their 
peculiarity is that they rise, for several 
successive evenings, almost at the same hour, 
immediately after sunset. This is due to the 
fact that at that time of the year the ecliptic, 
from which the moon's path does not very 
widely depart, is, in high latitudes, nearly 
parallel with the horizon. 

The full moon in winter runs higher in the 
sky, and consequently gives a brighter light, 
than in summer. The reason is because, 
since the full moon must always be opposite 
to the sun, and since toe sun in winter runs 
low, being south of the equator, the full moon 
rides proportionally high. 

5. Eclipses. We have mentioned the 
connection of the moon with the tides, but 
there is another phenomenon in which the 
moon plays the most conspicuous part — 
that of eclipses. There are two kinds of 
eclipses — solar and lunar. In the former 
it is the moon that causes the eclipse, by 
hiding the sun from view; and in the latter 
it is the moon that suffers the eclipse, by 
passing through the shadow which the earth 
casts into space on the side away from the 



164 THe Solar System 

sun. In both cases, in order that there may- 
be an eclipse it is necessary that the three 
bodies, the moon, the sun, and the earth, 
shall be nearly on a straight line, drawn 
through their centres. Since the moon 
occupies about a month in going round the 
earth there would be two eclipses in every 
such period (one of the sun and the other 
of the moon), if the moon's orbit lay exactly 
in the plane of the ecliptic, or of the earth's 
orbit. But, in fact, the orbit of the moon is 
inclined to that plane at an angle of some- 
thing over 5 . Even so, there would be 
eclipses every month if the two opposite 
points, called nodes, where the moon crosses 
the plane of the ecliptic, always lay in a 
direct line with the earth and the sun; but 
they do not lie thus. If, then, the moon 
comes between the earth and the sun when 
she is in a part of her orbit several degrees 
above or below the plane of the ecliptic, it 
is evident that she will pass either above or 
below the straight line joining the centres of 
the earth and the sun, and consequently 
cannot hide the latter. But, since eclipses 
do occur in some months and do not occur 
in others, we must conclude that the situa- 
tion of the nodes changes, and such is the 



Ellipses 165 

fact. In consequence of the conflicting 
attractions of the sun and the earth, the 
orbit of the moon, although, like that of 
the earth, it always retains nearly the same 
shape and the same inclination, swings round 
in space, so that the nodes, or crossing 
points on the ecliptic, continually change 
their position, revolving round the earth. 
This motion may be compared to that of 
the precession of the equinoxes, but it is 
much more rapid, a complete revolution 
occurring in a period of about nineteen years. 
From this it follows that sometimes the 
moon in passing its nodes will be in a line 
with the sun, and sometimes will not. But, 
owing to the fact that the sun and moon are 
not mere points, but on the contrary present 
to our view circular disks, each about half 
a degree in diameter, an eclipse may occur 
even if the moon is not in an exact line with 
the centres of the sun and the earth. The 
edge of the moon will overlap the sun, and 
there will be a partial eclipse, if the centres 
of the two bodies are within one degree 
apart. Now, the inclination of the moon's 
orbit to the ecliptic being only a little over 
5°, it is apparent that in approaching one of 
its nodes, along so gentle a slope, it will 



166 THe Solar System 

come within less than a degree of the ecliptic 
while still quite far from the node. Thus, 
eclipses occur for a considerable time before 
and after the moon passes a node. The 
distances on each side of the node, within 
which an eclipse of the sun may occur, are 
called the solar ecliptic limits, and they 
amount to i8° in either direction, or 36 in 
sum. Within these limits the sun may be 
wholly or partially eclipsed according as the 
moon is nearer to, or farther from, the node. 
If she is exactly at, or very close to, the node 
the eclipse will be total. 

Solar eclipses vary in another way. What 
would be a total eclipse, under other cir- 
cumstances, may be only an annular eclipse 
if the moon happens to be near her greatest 
distance from the earth. We have described 
the variations in her distance due to the 
eccentricity of her orbit, and we have said 
that the orbit itself swings round the earth 
in such a way as to cause the nodes continu- 
ally to change their places on the ecliptic. 
The motion of the orbit also causes the lunar 
apsides, or the points where she is at her 
greatest and least distances from the earth, 
to change their places, but their revolution 
is opposite in direction to that of the nodes, 



Eclipses 167 

as the revolution of the apsides of the earth's 
orbit is opposite to that of the equinoxes. 
The moon's apsides sometimes move east- 
ward and sometimes westward, but upon the 
whole the eastward motion prevails and the 
apsides complete one revolution in that direc- 
tion once in about nine and one-half years. 
In consequence of the combined effects 
of the revolution of the nodes and that of 
the apsides, the moon is sometimes at her 
greatest distance from the earth at the 
moment when she passes centrally over the 
sun, and sometimes at her least distance, 
or she may be at any intervening distance. 
If she is in the nearer part of her orbit, 
her disk just covers that of the sun, and the 
eclipse is total; if she is in the farther part 
(since the apparent size of bodies diminishes 
with increase of distance), her disk does not 
entirely cover the sun, and a rim of the 
latter is visible all around the moon. This 
is called an annular eclipse, because of the 
ring shape of the part of the sun remaining 
visible. 

The length of the shadow which the moon 
casts toward the earth during a solar eclipse 
also plays an important part in these phe- 
nomena. This length varies with the distance 



168 The Solar System 

from the sun. Since the moon accompanies 
the earth, it follows that when the latter 
is in aphelion, or at its greatest distance from 
the sun, the moon is also at its greatest 
mean distance from the sun, and the length 
of the lunar shadow may, in such circum- 
stances, be as much as 236,000 miles. When 
the earth, attended by the moon, is in 
perihelion, the length of the moon's shadow 
may be only about 228,000 miles. The 
average length of the shadow is about 232,000 
miles. This is nearly 7000 miles less than 
the average distance of the moon from the 
earth, so it is evident that generally the 
shadow is too short to reach the earth, and 
it would never reach it, and there would 
never be a total eclipse of the sun, but for 
the varying distance of the moon from the 
earth. When the moon is nearest the earth, 
or in perigee, its distance may be as small as 
221,600 miles, and in all cases when near 
perigee it is near enough for the shadow to 
reach the earth. 

Inasmuch, as the moon's shadow, even 
under the most favourable circumstances, 
is diminished almost to a point before touch- 
ing the earth, it hardly need be said that it 
can cover but a very small area on the earth's 



Eclipses 169 

surface. Its greatest possible diameter can- 
not exceed about 167 miles, but ordinarily it 
is much smaller. If both the earth and the 
moon were motionless, this shadow would be a 
round or oblong dot on the earth, its shape 
varying according as it fell square or sloping 
to the surface ; but since the moon is continu- 
ally advancing in its orbit, and the earth is 
continually rotating on its axis, the shadow 
moves across the earth, in a general west 
to east direction. But the precise direction 
varies with circumstances, as does also the 
speed. The latter can never be less than 
about a thousand miles per hour, and that, 
only in the neighbourhood of the equator. 
The moon advances eastward about 2100 
miles per hour, and the earth's surface turns 
in the same direction with a velocity dimin- 
ishing from about a thousand miles an hour 
at the equator to o at the poles. It is the 
difference between the velocity of the earth's 
rotation and that of the moon's orbital 
revolution which determines the speed of 
the shadow. The greatest time, which the 
shadow can occupy in passing a particular 
point on the earth is only eight minutes, but 
ordinarily this is reduced to one, two, or 
three minutes. The true shadow only lasts 



170 THe Solar System 

during the time that the moon covers the 
whole face of the sun, but before and after 
this total obscuration of the solar disk the 
sun is seen partially covered by the moon, 
and these partial phases of the eclipse may 
be seen from places far aside from the track 
which the central shadow pursues. It is 
only during a total eclipse, and only from 
points situated within the shadow track, that 
the solar corona is visible. 

In a lunar eclipse it is the earth that is 
the intervening body, and its shadow falls 
upon the moon. A solar eclipse can only 
occur at the time of new moon, and a lunar 
eclipse only at the time of full moon. The 
shadow of the earth is much longer and 
broader than that of the moon, and it never 
fails to reach the moon, so that it is not 
necessary here to consider its varying length. 
The width, or diameter, of the shadow at 
the average distance of the moon from the 
earth is about 5700 miles. The moon may 
pass through the centre of the shadow, or 
to one side of the centre, or merely dip into 
the edge of it. When it goes deep enough 
into the shadow to be entirely covered, 
the eclipse is total; otherwise it is partial. 
Since in a total lunar eclipse the entire moon 



Eclipses 171 

is covered by the shadow, it is evident that 
such an eclipse, unlike a solar one, may be 
visible simultaneously from all parts of the 
earth which, at the time, lie on the side 
facing the moon. In other words, the earth's 
shadow does not make merely a narrow 
track across the face of the moon, but 
completely buries it. When the moon passes 
centrally through the shadow, she may 
remain totally obscured for about two hours. 
But the moon does not completely dis- 
appear at such times, because the refraction 
of the earth's atmosphere bends a little 
sunlight round its edges and casts it into 
the shadow. If the atmosphere round the 
edges of the earth happens to be thickly 
charged with clouds, but little light is thus 
refracted into the shadow, and the moon 
appears very faint, or almost entirely dis- 
appears. But this is rare, and ordinarily 
the eclipsed moon shines with a pale cop- 
perish light. 

The occurrence of a lunar eclipse is governed 
by similar circumstances to those affecting 
solar eclipses. The lunar ecliptic limits, 
or the distance on each side of the node 
within which an eclipse may occur, vary 
from 9^° to 12^° in either direction. 



172 THe Solar System 

Taking all the various circumstances into 
account, it is found that there may be, 
though rarely, seven eclipses in a year, 
two being of the moon and five of the sun, 
and that the least possible number of eclipses 
in a year is two, in which case both will be 
of the sun. Taking into account also all 
the various positions which the sun and moon 
occupy with regard to the earth, it is found 
that there exists a period of 18 years, n 
days, 8 hours, at the return of which eclipses 
of both kinds begin to recur again in the 
same order that they occur in the next 
preceding period. This is called the saros, 
and it was known to the Chaldeans 2600 
years ago. 

6. The Planets. We have several times 
mentioned the fact that, beside the earth, 
there are seven other principal planets 
revolving round the sun, in the same direction 
as the earth, but at various distances. 
We shall consider each of these in the order of 
its distance from the sun. 

But first it is desirable to explain briefly 
certain so-called "laws" which govern the 
motions of all the planets. These are known 
as Kepler's laws of planetary motion, and 
are three in number. The demonstration 



The Planets 173 

of their truth would carry us beyond the 
scope of this book, and consequently we 
shall merely state them as they are recognised 
by astronomers. 

ist Law: The orbit of every planet is an 
ellipse, having the sun situated in one of the 
foci. 

2d Law: The radius vector of a planet 
describes equal areas in equal times. By 
the radius vector is meant the straight line 
joining the planet to the sun, and the law 
declares that as the planet moves round the 
sun, the area of space swept over by this 
line in any given time, say one day, is equal 
to the area which it will sweep over in any 
other equal length of time. If the orbit 
were a circle it is evident at a glance that the 
law must be true, because then the sun would 
be situated in the centre of the circle, the 
length of the radius vector, no matter where 
the planet might be in the orbit, would never 
vary, and the area swept over by it in one day 
would be equal to the area swept over in 
any other day, because all these areas would 
be precisely similar and equal triangles. 
But Kepler discovered that the same thing 
is true when the orbit is an ellipse, and when, 
in consequence of the eccentricity of the 



174 THe Solar System 

orbit, the planet is sometimes farther from 
the sun than at other times. As the trian- 
gular area swept over in a given time in- 
creases in length with the planet's recession 
from the sun, it diminishes in breadth just 
enough to make up the difference which 
would otherwise exist between the different 
areas. This law grows out of the fact that 
the force of gravitation varies inversely with 
the square of the distance. 

3d Law: The squares of the periods (i. e, 
times of revolution in their orbits) of the 
different planets are proportional to the cubes 
of their mean (average) distances from the 
sun. The meaning of this will be best 
explained by an example. Suppose one 
planet, whose distance we know, has a period 
only one-eighth as long as that of another 
planet, whose distance we do not know. 
Then Kepler's third law enables us to cal- 
culate the distance of the second planet. 
Call the period of the first planet 1 , and that 
of the second 8, and also call the distance of 
the first 1, since all we really need to know 
is the relative distance of the second, from 
which its distance in miles is readily deduced 
by comparison with the distance of the first. 
Then, by the law, I 2 : 8 2 : I 3 : x 3 ("x" 



The Planets 175 

representing the unknown quantity). Now, 
this is simply a problem in proportion where 
the product of the means is equal to the 
product of the extremes. But 1 2 = i , and also 
i 3 = i; therefore x 3 = 8 2 , and x = l^8 2 (the 
cube root of the square of 8), which is 4. 
Thus we see that the distance of the second 
planet must be four times that of the first. 

This third law of Kepler is applied to 
ascertain the distances of newly discovered 
planets, whose periods are easily ascertained 
by simple observation. If we know the 
distance of any one planet by measurement, 
we can calculate the distances of all the 
others after observing their periods. The 
law also works conversely, i. e. from the 
distances the periods can be calculated. 

We now take up the various planets singly. 
The nearest to the sun, as far as known, 
is Mercury, its average distance being only 
36,000,000 miles. But its orbit is so eccen- 
tric that the distance varies from 28,500,000 
miles at perihelion to 43,500,000 at aphelion. 
In consequence its speed in its orbit is very 
variable, and likewise the amount of heat 
and light received by it from the sun. On 
the average it gets more than 6^ times 



176 THe Solar System 

as much solar light and heat as the earth 
gets. But at perihelion it gets 2^ times 
as much as at aphelion, and the time which 
it occupies in passing from perihelion to 
aphelion is only six weeks, its entire year 
being equal to 88 of our days. Being situ- 
ated so much nearer the sun than the 
earth is, Mercury is never visible to us 
except in the morning or the evening sky, 
and then not very far from the sun. Its 
diameter is about 3000 miles, but its mass 
is not certainly known from lack of know- 
ledge of its mean density. This lack of 
knowledge is due to the fact that Mercury 
has no satellite. When a planet has a satel- 
lite it is easy to calculate its density from its 
measured diameter combined with the orbital 
speed of its satellite. Certain considerations 
have led some to believe that the mean density 
of Mercury may be very great, perhaps as 
great as that of lead, or of the metal mercury 
itself. Not knowing the mass, we cannot 
say exactly what the weight of bodies on 
Mercury is. We are also virtually ignorant 
of the condition of the surface of this planet, 
the telescope revealing very little detail, 
but it is generally thought that it bears a 
considerable resemblance to the surface of 



The Planets 177 

the moon. There is another way in which 
Mercury is remarkably like the moon. The 
latter, as we have seen, always keeps the 
same side turned toward the earth, which 
is the same thing as saying that it turns once 
on its axis, while going once around the 
earth. So Mercury keeps always the same 
side toward the sun, making one rotation on 
its axis in the course of one revolution in 
its orbit. Consequently, one side of Mer- 
cury is continually in the sunlight, while the 
opposite side is continually buried in night. 
There must, however, be regions along the 
border between these two sides, where the 
sun does rise and set once in the course of one 
of Mercury's years. This arises from the 
eccentricity of the orbit, and the consequent 
variations in the orbital velocity of the planet, 
which cause now a little of one edge and now 
a little of the other edge of the dark hemi- 
sphere to come within the line of sunlight. 
(The same thing occurs with the moon, though 
to a less degree owing to the smaller eccen- 
tricity of the moon's orbit, which, however, 
is sufficient to enable us to see at one time 
a short distance round one side of the moon 
and at another time a short distance round 
the opposite side.) This phenomenon is 



178 THe Solar System 

known as libration. Mercury apparently 
possesses an atmosphere, but we know 
nothing certain concerning its density. 

The next planet, in the order of distance 
from the sun, is Venus, whose average 
distance is 67,200,000 miles. The orbit 
of Venus is remarkable for its small eccen- 
tricity, so that the difference between its 
greatest and least distances from the sun is 
less than a million miles. The period, or 
year, of Venus is 225 of our days. Owing 
to her situation closer to the sun, she gets 
nearly twice as much light and heat as the 
earth gets. In size Venus is remarkably 
like the earth, her diameter being 7713 
miles, which differs by only 205 miles from 
the mean diameter of the earth. Her axis 
is nearly perpendicular to the plane of her 
orbit. Her globe is a more perfect sphere 
than that of the earth, being very little flat- 
tened at the poles or swollen at the equator. 
Although Venus, like Mercury, has no 
satellite, her mean density has been calcu- 
lated by other means, and is found to be 
0.89 that of the earth. From this, in con- 
nection with her measured diameter, it is 
easy to deduce her mass, and the force of 



The Planets 179 

gravity on her surface. The latter comes out 
at about 0.85 that of the earth, i. e. a body 
weighing 100 pounds on the earth would 
weigh 85 pounds if removed to Venus. She 
possesses an atmosphere denser and more 
extensive than would theoretically have been 
expected — indicating, perhaps, a difference 
of constitution. Her atmosphere has been 
estimated to be twice as dense as ours, a 
great advantage, it may be remarked, from 
the point of view of aeronautics. But this 
dense and abundant atmosphere renders 
Venus a very difficult object for the telescope 
on account of the brilliance of its reflection. 
In consequence, we know but little of the 
surface of the planet. 

One important result of this is that the 
question remains undecided whether Venus 
rotates on her axis at a rate closely corre- 
sponding with that of the earth, as some 
observers think, or whether, as others think, 
she, like Mercury, turns only once on her 
axis in going once round the sun. The 
importance of the question in its bearing 
on the habit ability of Venus is apparent, 
for if she keeps one face always sunward, 
then on one side there is perpetual day and 
on the other perpetual night. On the 



180 The Solar System 

other hand, if she has days and nights 
approximately equal in length to those of 
the earth, it may well be thought that she 
is habitable by beings not altogether unlike 
ourselves, because the force of gravity on her 
surface is not much less than on the earth, 
and her dense atmosphere, filled with clouds, 
might tend to shield her inhabitants from 
the effects of the greater amount of heat 
poured upon her by the sun. As her orbit 
is inside that of the earth, Venus, like Mer- 
cury, is only visible either in the evening 
or in the morning sky, but owing to her 
greater actual distance from the sun, her 
apparent distance from it in the sky is 
greater than that of Mercury. 

Both of these planets, in consequence of 
passing alternately between the sun and the 
earth and round the opposite side of the sun, 
present phases resembling those of the moon. 
The reader can explain these to himself by 
means of the experiment, before mentioned, 
with a billiard ball and a lamp. In this 
case let the observer remain seated in his 
chair while another person carries the ball 
round the lamp in such a manner that it 
shall alternately pass between the lamp and 
the observer and round the other side of the 



The Great Nebula in Orion 

Photographed at the Lick Observatory by J. E. Keeler, with the Crossley 
reflector. Exposure one hour. 



The Planets 181 

lamp. When Venus comes nearly in line 
between the earth and the sun, she becomes 
an exceedingly brilliant object in either the 
evening or the morning sky, although at 
such times we see, in the form of a crescent, 
only a part of that half of her surface which 
is illuminated. Her increase of brightness 
at such times is due to her greater nearness 
to the earth. When between the earth and 
the sun she may be only about 26,000,000 
miles away, while when she is on the other 
side of the sun she may be over 160,000,000 
miles away. Both Venus and Mercury 
when passing exactly between the sun and 
the earth are seen, in the form of small black 
circles, moving slowly across the sun's disk. 
These occurrences are called transits, and 
in the case of Venus have been before referred 
to. They are more frequent with Mercury 
than with Venus, but Mercury's transits 
are not utilisable for parallax observations. 
The latest transit of Venus occurred in 
1882, and there will not be another until 
2004. The latest transit of Mercury occurred 
in 1907, and there will be another in 19 14. 

The earth is the third planet in order of 
distance, and then comes Mars, whose 



1 82 THe Solar System 

average distance from the sun is 141,500,000 
miles. The orbit of Mars is so eccentric 
that the distance varies between 148,000,000 
and 135,000,000 miles. Its period or year 
is about 687 of our days. In consequence 
of its distance, Mars gets, on the average, 
a little less than half as much light and heat 
as the earth gets. When it is on the same 
side of the sun with the earth, and nearly in 
line with them, it is said to be in opposition. 
At such times it is manifestly as near the 
earth as it can come, and thus an opposition 
of Mars offers a good opportunity for the 
telescopic study of its surface. These op- 
positions occur once in about 780 days, 
but they are not all of equal importance, 
because the distance between the two planets 
is not the same at different oppositions. 
The cause of the difference of distance is 
the eccentricity of the orbit. If an opposi- 
tion occurs when Mars is in aphelion its dis- 
tance from the earth will be about 61,000,000 
miles, but if the opposition occurs when 
Mars is in perihelion the distance will be 
only about 35,000,000 miles. The average 
distance at an opposition is about 48,500,000 
miles. The most favourable oppositions 
always occur in August or September, 



Tine Planets 



183 



and are repeated at an interval of from 
fifteen to seventeen years. But at some of 



APHELION 




Noy.ssjpn 



WSEPT.Z^igOQ. 



PERIHELION 



^^s^o^r 



Fig. 16. Orbits of Mars and the Earth. 

Inspection shows at once why the oppositions of Mars 
which occur in August and September are the most 
favourable because Mars being then near the perihelion 
point of its elongated orbit is comparatively near the 
earth, while oppositions which occur in February and 
March are very unfavourable because then Mars is near 
the aphelion point of its orbit, and its distance from the 
earth is much greater. The oppositions occur along the 
more favourable part of the orbit about two years and 
two months apart. Thus the figure shows that the 
opposition of September 24, 1909 was followed by one 
on November 25, 191 1. 

the intervening oppositions the distance of 
the planet is not too great to afford good 
views of its surface. The diameter of Mars 



184 TKe Solar System 

is about 4330 miles, with a similar polar 
flattening to that of the earth. Its density- 
is 0.71 that of the earth, and the force of 
gravity on its surface 0.38. A body weighing 
100 pounds on the earth would weigh 38 
pounds on Mars. The evidence in regard 
*to its atmosphere is conflicting, but the 
probability is that it has an atmosphere 
not denser than that existing on our highest 
mountain peaks. Opinions concerning the 
existence of water-vapour on Mars are also 
conflicting. One fact tending to show that 
its atmosphere must be very rare and cloud- 
less is that its surface features are very 
plainly discernible with telescopes. 

About each pole, as it happens to be turned 
earthward, is to be seen a round white patch 
(supposed to be snow), and this gradually 
disappears as the summer advances in that 
hemisphere of the planet — for Mars has 
seasons very closely resembling our seasons, 
except that they are about twice as long. 
The inclination of the axis of Mars to the 
plane of its orbit is about 24 50', which is 
not very different from the inclination of 
the earth's axis. Moreover, Mars rotates 
in a period of 24 hours, 37 min., 22 sec, 
so that the length of day and night upon 



The Planets 185 

its surface is very nearly the same as upon 
the earth. The surface of the planet is 
marked by broad irregular areas of con- 
trasting colour, or tone, some of them being 
of a slightly reddish, or yellowish, hue, 
and others of a neutral dusky tint. The 
general resemblance to a globe of the earth, 
with differently shaped seas and oceans, 
is striking. 

On account of the many likenesses between 
Mars and the earth, some astronomers 
are disposed to think that Mars may be a 
habitable planet. The terms ''seas" and 
"continents" were formerly applied to the 
contrasted areas just spoken of, but now it 
is believed that there are no large bodies of 
water on Mars. Crossing the light, or 
reddish-coloured, areas there are sometimes 
seen great numbers of intersecting lines, 
very narrow and faint, which have received 
the name of "canals." Some speculative 
minds find in these ground for believing that 
they are of artificial origin, and a theory 
has been built up, according to which the 
so-called canals are "irrigated bands," the 
result of the labours of the inhabitants. 
The argument of the advocates of this theory 
is put about as follows: Mars is evidently 



186 The Solar System 

a nearly dried-up planet, and most of the 
water left upon it is periodically locked up 
in the polar snows. As these snows melt 
away in the summer time, now in one hemi- 
sphere and now in the other, the water thus 
formed is conducted off toward the tropical 
and equatorial zones by innumerable canals, 
too small to be seen from the earth. The 
lands irrigated by these canals are narrow 
strips, whose situation is determined by 
local circumstances, and which cross one 
another in all directions. Within these 
bands, which enlarge into rounded "oases" 
where many of them intersect, vegetation 
pushes, and its colour causes them to appear 
as dark lines and patches on the surface 
of the planet. The fact that the lines make 
their appearance gradually, after the polar 
caps begin to disappear, is regarded as 
strongly corroborative of the theory. In 
answer to the objection that works so 
extensive as this theory of irrigation calls 
for would be practically impossible, it is 
replied that the relatively small force of 
gravity on Mars not only immensely di- 
minishes the weight of all bodies there, but 
also renders it possible for animal forms 
to attain a greater size, with corresponding 



The Planets 187 

increase of muscular power. It is likewise 
argued that Mars may have been longer 
inhabited than the earth, and that its in- 
habitants may consequently have developed 
a more complete mastery over the powers 
of nature than we as yet possess. Many 
astronomers reject these speculations, and 
even aver that the lines called " canals" 
(and it must be admitted that many powerful 
telescopes show few or none of them) 
have no real existence, what is seen, or 
imagined to be seen, being due to some 
peculiarity of the soil, rocks, or atmosphere. 
Mars has two small satellites, revolving 
round it with great speed at close quarters. 
The more distant satellite, Deimos, is 14,600 
miles from the centre of Mars and goes 
round it in 30 hours, 18 min. The nearer 
one, Phobos, is only 5800 miles from the 
planet's centre, and its period of revolution 
is only 7 hours, 39 min., so that it makes 
more than three circuits while the planet 
is rotating once on its axis. Both of the 
satellites are minute in size, probably under 
ten miles in diameter. 

Beyond Mars, at an average distance of 
about 246,000,000 miles from the sun, is a 



188 The Solar System 

system of little planets called asteroids. 
More than 600 are now known, and new ones 
are discovered every year, principally by 
means of photography. Only four of these 
bodies are of any considerable size, and they 
were, naturally, the first to be discovered. 
They are Ceres, diameter 477 miles; Pallas, 
304 miles; Vesta, 239 miles; and Juno, 120 
miles. Many of the others have a diameter 
of only about ten, or even, perhaps, as little 
as five, miles. Their orbits are more ec- 
centric than those of any of the large planets, 
and one of them, Eros, has a mean distance of 
135,000,000 miles, and a least distance of only 
105,000,000, so that it is nearer to the sun 
than Mars is. Eros may, under favourable 
circumstances, approach within 14,000,000 
miles of the earth. This fact, as already 
mentioned, has been taken advantage of 
for measuring its distance from the earth, 
from which the distance of the sun may be 
calculated with increased accuracy. Eros 
and some others of the asteroids seem to be 
of an irregular or fragmentary form, and 
this has been used to support a theory, which 
is not, however, generally accepted, that the 
asteroids are the result of an explosion, 
by which a larger planet was blown to pieces. 



TKe Planets 189 

Sixth in order of distance from the sun 
(counting the asteroids as representing a 
single body) is the greatest of all the planets, 
Jupiter. His average distance from the sun 
is 483,000,000 miles, but the eccentricity 
of his orbit causes him to approach within 
472,500,000 miles at perihelion, and to recede 
to 493,500,000 miles at aphelion. When 
in opposition, Jupiter's mean distance from 
the earth is 390,000,000 miles. This gigantic 
planet has a mean diameter of 87,380 miles, 
but is so flattened at the poles and bulged 
round the equator that the polar diameter 
is only 84,570 miles, while the equatorial 
diameter is 90,190 miles, a difference of 
5680 miles. This peculiar form is doubtless 
due to the planet's swift rotation. The 
axis, like that of Venus, is nearly perpendicu- 
lar to the plane of the orbit. He makes a 
complete turn on his axis in a mean period 
of 9 hours, 55 minutes. The reason for saying 
"a mean period" will appear in a moment. 
Jupiter's year is equal to 11.86 of our years, 
but it comes into opposition to the sun, as 
seen from the earth, once in every 399 days. 

The volume of Jupiter is about 1300 times 
that of the earth, i.e. it would take 1300 
earths rolled into one to equal Jupiter in 



190 TKe Solar System 

size. But its mean density is slightly less 
than one quarter of the earth's, so that its 
mass is only 316 times greater than the 
earth's. The force of gravity on its surface 
is 2.64 times the earth's. A body weighing 
100 pounds on the earth would weigh 264 
pounds on Jupiter. It will be observed that 
Jupiter's mean density is very nearly the 
same as that of the sun, and we conclude that 
it cannot be a solid, rigid globe like the earth. 
This conclusion is made certain by the fact 
that its period of rotation on its axis is 
variable, another resemblance to the sun. 
The equatorial parts go round in a shorter 
period than parts situated some distance 
north or south of the equator. It may be 
supposed that there is a solid nucleus within, 
but if so, no direct evidence of its existence 
has been found. 

Nevertheless, although Jupiter appears to 
be in a cloud-like state, it does not shine 
with light of its own, so that its temperature, 
while no doubt higher than that of the earth, 
cannot approach anywhere near that of the 
sun. We do not know of what materials 
Jupiter is composed, for spectroscopic analy- 
sis applies especially to bodies which shine 
with their own light. When they shine only 



The Planets 191 

by reflected light received from the sun, 
their spectra resemble the regular solar 
spectrum, except for the presence of faint 
bands due to absorption in the planet's 
atmosphere. It may be that there are no 
elements of great atomic density, such as 
iron or lead, in the globe of Jupiter. Yet 
in the course of long ages the planet may 
become smaller and more condensed, in 
consequence of the escape of its internal 
heat. In this way Jupiter may be regarded 
as representing an intermediate stage of 
evolution between an altogether vaporous 
and very hot body like the sun, and a cool 
and solid one like the earth. 

Jupiter presents a magnificent appearance 
in a good telescope. Its oblong disk is 
seen crossed in an east and west direction, 
and parallel to its equator, by broad, vari- 
coloured bands, called belts. These fre- 
quently change in form and, to some extent, 
in situation, as well as in number. But 
there are always at least two wide belts, 
one on each side of the equator. In 1878 
a very remarkable feature was noticed just 
south of the principal south belt of Jupiter, 
which has become celebrated under the name 
of the Great Red Spot. In a few years 



192 Trie Solar System 

after its discovery its colour faded, but it 
still remains visible, with varying degrees 
of distinctness, as an oblong marking, about 
30,000 miles long and 7000 miles broad. 
The outer border of the great south belt 
bends away from the spot, as if some force 
of repulsion acted between them, or as if the 
spot were an elevation round which the 
clouds of the belt flowed like a river round 
a projecting headland. The nature of this 
curious spot is unknown. Other smaller 
spots, sometimes white, sometimes dusky, 
occasionally make their appearance, but 
they do not exhibit the durability of the 
Great Red Spot. 

Jupiter has eight satellites, four of which, 
known since the time of Galileo, are con- 
spicuous objects in the smallest telescope. 
All but one of these four are larger than our 
moon, while the other four are extremely 
insignificant in size. The four principal 
satellites are designated by Roman numerals, 
I, II, III, IV, arranged in the order of dis- 
tance from the planet. They also have names 
which are seldom used. Satellite I (Io) 
has a diameter of 2452 miles, and revolves 
in a period of 1 day, 18 hours, 27 min., 35.5 
sec, at a mean distance of 261,000 miles; 



I 



The Planets 193 

II (Europa) is 2045 miles in diameter, 
and revolves in 3 days, 13 hours, 13 min., 
42.1 sec., at a mean distance of 415,000 
miles; III (Ganymede) has a diameter of 
3558 miles, a period of seven days, 3 hours, 
42 min., 33.4 sec., and a mean distance of 
664,000 miles; IV (Callisto) is 3345 miles 
in diameter, has a period of 16 days, 16 hours, 
32 min., 1 1.2 sec, and a mean distance of 
1,167,000 miles. The object of giving the 
periods with extreme accuracy will appear 
when we speak of the use made of obser- 
vations of Jupiter's satellites. The first of 
the four small satellites, discovered by 
Barnard in 1892, is probably less than 100 
miles in diameter, and has a mean distance 
of 112,500 miles, and a period of only 11 
hours, 57 min., 22.6 sec The other small 
satellites are much more distant than any 
of the large ones, the latest to be discovered, 
the eighth, being situated at a mean distance 
of about 15,000,000 miles, but travelling 
in an orbit so eccentric that the distance 
ranges between 10,000,000 and 20,000,000 
miles. The period is about two and a fifth 
years. But the most remarkable fact is 
that this satellite revolves round Jupiter 
from east to west, a direction contrary to 
13 



194 THe Solar System 

that pursued by all the others, and contrary 
to the direction which is almost universal 
among the rotating and revolving bodies of 
the solar system. 

The large satellites are very interesting 
objects for the telescope. When they come 
between the sun and Jupiter their round black 
shadows can be plainly seen moving across 
his disk, and when they pass round into his 
shadow they are suddenly eclipsed, emerging 
after a time out of the other side of the shadow. 
These phenomena are known as transits and 
eclipses, and their times of occurrence are 
carefully predicted in the American Epheme- 
ris and Nautical Almanac, published at 
Washington for the benefit of astronomers 
and navigators, because these eclipses can 
be employed in comparing local time with 
standard meridian time. They were form- 
erly utilised to determine the velocity of 
light, in this way : 

As the earth goes round its orbit inside 
that of Jupiter the latter is seen in opposition 
to the sun at intervals of 399 days. When 
it is thus seen the earth must be between 
the sun and Jupiter, and the distance between 
the two planets is the least possible. But 
when the earth has passed round to the other 



The Planets 195 

side of the sun from Jupiter this distance 
becomes the greatest possible. The increase 
of distance between the two planets, as the 
earth goes from the nearest to the farthest 
side of its orbit, is about 186,000,000 miles. 
Now it was noticed by the Danish astro- 
nomer, Roemer, that as the earth moved 
farther and farther from Jupiter the times 
of occurrence of the eclipses kept getting 
later and later, until when the earth arrived 
at its greatest distance the eclipses were 
about 16 minutes behind time. He correctly 
inferred that the retardation of the time was 
due to the increase of the distance, and that 
the 16 minutes by which the eclipses were 
behindhand when the distance was greatest 
represented the time taken by light to cross 
the 186,000,000 miles of space by which the 
earth had increased its distance from Jupiter. 
In other words, light must travel 186,000,000 
miles in about sixteen minutes, from which 
it was easy to calculate its speed per second 
— which we now know to be 186,330 miles. 
Our knowledge of the velocity of light fur- 
nishes one of the means of calculating the 
distance of the sun. 

We come next to the beautiful planet 
Saturn, whose mean distance from the sun 



196 THe Solar System 

is 886,000,000 miles. The distance varies 
between 911,000,000 and 861,000,000 miles. 
Saturn's year is equal to 29.46 of our years. 
It comes into opposition every 378 days. 
The most surprising feature of Saturn is 
the system of immense rings surrounding 
it above the equator. The globe of the 
planet is 76,470 miles in equatorial diameter, 
and 69,780 miles in polar diameter, a differ- 
ence of 6690 miles, so that Saturn is even 
more compressed at the poles and swollen 
at the equator than Jupiter. The axis of 
rotation is inclined 27 from a perpendicular. 
The rings are three in number, very thin 
in proportion to their vast size, and placed 
one within another in the same plane. The 
outer diameter of the outer ring, called Ring 
A, is about 168,000 miles. Its breadth is 
about 10,000 miles. Then comes a gap, 
about 1600 miles across, separating it from 
Ring B, the brightest of the set. This is 
about 16,500 miles broad, and at its inner 
edge it gradually fades out, blending with 
Ring C, which is called the crape, or gauze, 
ring, because it has a dusky appearance, 
and is so translucent that the globe of the 
planet can be seen through it. This ring is 
about 10,000 miles broad, and its inner edge 



The Planets 197 

comes within a distance of between 9000 
and 10,000 miles of the surface of the planet. 
Ring A apparently has a very narrow gap 
running round at about a third of its breadth 
from the outer edge. This, known as 
Encke's Division, is not equally plain at all 
times. Occasionally observers report the 
temporary appearance of other thin gaps. 
The mean density of Saturn is less than 
that of any other planet, being but 0.13 that 
of the earth, or 0.72 that of water. It 
follows that this great planet would float in 
water. The weight of bodies at its surface 
would be a little less than three quarters 
of their weight on the surface of the earth. 
The globe of Saturn, like that of Jupiter, 
is marked by belts parallel with the equator, 
but they are less definite in outline and less 
conspicuous than the belts of Jupiter. The 
equatorial zone often shows a beautiful pale 
salmon tint, while the regions round the 
poles are faintly bluish. Light spots are 
occasionally seen upon the planet, and it 
appears to rotate more rapidly at the equator 
than in the higher latitudes. There seems 
to be every reason to think that Saturn, 
also, is of a vaporous constitution, although 
it may have a relatively condensed nucleus. 



198 THe Solar System 

But while the globe of the planet appears 
to be vaporous, the same is not true of the 
rings. We have already mentioned the fact 
that they are exceedingly thin in proportion 
to their great size and width. The thickness 
has not been determined with exactness, 
but it probably does not exceed, on the aver- 
age, one hundred miles. There appear to be 
portions of the rings which are thicker than 
the average, as if the matter of which they 
are composed were heaped up there. This 
matter evidently consists of an innumerable 
multitude of small bodies. In other words, 
the rings are composed of swarms of what 
may be called meteors. That their com- 
position must be of this nature, although 
the telescope does not reveal it, has been 
proved in two ways: first, by mathematical 
calculation, which shows that if the rings 
were all of a piece, whether solid or liquid, 
they would be destroyed by the contending 
forces of attraction to which they are subject ; 
and, second, by spectroscopic observation, 
which proves, in a way that will be shown 
when we come to deal with the stars, that 
the rings rotate with velocities proportional' 
to the distances of their various parts from 
the centre of the planet. Hence it is inferred 



The Planets 199 

that they must consist of a vast number 
of small bodies or particles. 

Saturn has ten satellites, all revolving 
outside the rings. The names of nine of 
these in the order of increasing distance are: 
Mimas, distance 117,000 miles; Enceladus, 
distance 157,000 miles; Tethys, distance 
186,000 miles; Dione, distance 238,000 miles; 
Rhea, distance 332,000 miles; Titan, dis- 
tance 771,000 miles; Hyperion, distance 
934,000 miles; Japetus, distance 2,225,000 
miles; and Phoebe, distance 8,000,000 miles. 
The last, like the eighth satellite of Jupiter, 
revolves in a retrograde direction. Only 
Titan and Japetus are conspicuous ob- 
jects. The period of Mimas is only about 
223^2 hours; that of Titan is 15 days, 22 
hours, 41 min., and that of Japetus about 
79 days, 8 hours. Barnard's measurements 
indicate for Titan a diameter of 2720 miles. 
Japetus is probably about two-thirds as 
great in diameter as Titan. 

Beyond Saturn, in the order named, are 
Uranus and Neptune. The mean distance 
of the former from the sun is 1,782,000,000 
miles, and that of the latter 2,791,500,000 
miles. The orbit of Uranus is more eccentric 
than that of Neptune. The diameter of 



200 THe Solar System 

Uranus is about 32,000 miles and that of 
Neptune about 35,000 miles. The year of 
Uranus is equal to 84 of our years, and that 
of Neptune to 164.78. These planets are 
so remote, and so poorly illuminated by the 
sun, that the telescope reveals very little 
detail on their surfaces. Their density is 
somewhat less than that of Jupiter. Uranus 
has four satellites, Ariel, Umbriel, Titania, and 
Oberon, situated at the respective distances 
of 120,000, 167,000, 273,000, and 365,000 
miles. Neptune has one, nameless, satellite, 
at a distance of 225,000 miles. 

The most remarkable thing about these 
two planets is that their axes of rotation, 
as compared with those of all the other 
planets, are tipped over into a different 
plane, so that they rotate in a retrograde 
or backward direction, and their satellites, 
in like manner, revolve from east to west. 
The axis of Uranus is not far from upright 
to the plane of the ecliptic, so that the motion 
of its satellites carries them alternately far 
northward and far southward of that plane, 
but the axis of Neptune is tipped so far over 
that the retrograde, or east to west, motion 
is very pronounced. Neptune is celebrated 
for having been discovered by means of 




Photographs of Mars 

Made at the Yerkes Observatory by E. E. Barnard, with the forty-inch 
refractor, September 28, 1909. 



Comets 201 

mathematical calculations, based on its dis- 
turbing attraction on Uranus. These cal- 
culations showed where it ought to be at 
a certain time, and when telescopes were 
pointed at the indicated spot the planet was 
found. Similar disturbances of the motions 
of Neptune lead some astronomers to think 
that there is another, yet undiscovered, 
planet still more distant. 

7. Comets. Comets are the most ex- 
traordinary in appearance of all celestial 
objects visible to the naked eye. Great 
comets have been regarded with terror and 
superstitious dread in all ages of the world, 
wherever ignorance of their nature has 
prevailed. They have been taken for prog- 
nosticators of wars, famines, plagues, the 
death of rulers, the outbreak of revolutions, 
and the subversion of empires. One reason 
for this, aside from their strange and menac- 
ing appearance, is, no doubt, the rarity 
of very great and conspicuous comets. It 
was not until Newton had demonstrated the 
law of gravitation that the fact began to be 
recognised that comets are controlled in their 
motions by the sun. We now know that 
they travel in orbits, frequently, and perhaps 
always, elliptical, having the sun in one of 



202 TKe Solar System 

the foci. Comets are habitually divided 
into two classes: first, periodical comets, 
meaning those which have been observed 
at more than one return to perihelion ; and, 
second, non-periodical comets, meaning those 
which have been seen but once, but which, 
nevertheless, may return to perihelion in a 
period so long that a second return has not 
been observed. A better division is into 
comets of short period, and comets of long, 
or unknown, periods. 

Still, many astronomers are disposed to 
think that the majority of comets do not 
travel in elliptical but in parabolic, and a 
few in hyperbolic, orbits. This calls for a few 
words of explanation. Ellipses, parabolas, 
and hyperbolas are all conic-section curves, 
but the ellipse alone returns into itself, or 
forms a closed circuit. In each case the sun 
is situated at the focus where the perihelion, 
or nearest approach, of the comet occurs, 
but only comets travelling in elliptical 
orbits return again after having once been 
seen. A comet moving in a parabola would 
go back into the depths of space nearly in 
the direction from which it had come, and 
would never be seen again ; and if it moved in 
a hyperbola it would go off toward another 



Comets 



203 



quarter of the celestial sphere, and likewise 
would never return. Now it is true that 




Fig. 17. Ellipse, Parabola, and Hyperbola. 

The figure shows graphically why it is so difficult to tell 
exactly the form of a comet's orbit. The three kinds of 
curves are nearly of the same form near the focus (the 
Sun), and it is only in that part of its orbit that the 
comet can be seen. Moreover a comet is, at best, a 
misty and indefinite object, which renders it so much 
the more difficult to obtain good observations of its 
precise position and movement. 



the forms calculated for the orbits of the 
majority of comets that have been observed 
appear to be parabolic (a very few seem 
to be hyperbolic), and if this is the fact such 



204 THe Solar System 

comets cannot be permanent members of 
the solar system, but must enter it from far- 
off regions of space, and having visited the 
sun must return to such regions without any 
tendency to come back again. In that case 
they may pay similar visits to other suns. 
But it is quite possible that what appear 
to be parabolic orbits may, in reality, be 
ellipses of very great eccentricity. The 
difficulty in determining the precise shape of 
a comet's orbit arises from the fact that all 
three of the curves just mentioned closely 
approximate to one another in the neigh- 
bourhood of their common focus, the sun, 
and it is only in that part of their orbits 
that comets are visible. The whole question 
is yet in abeyance, but, as we have said, 
it seems likely that all comets really move 
in elliptical orbits, and consequently never 
get entirely beyond the control of the sun's 
attraction. But in all cases the orbits of 
comets are much more eccentric than those 
of the large planets. The famous comet of 
Halley, for instance, which has the longest 
period of any of the known periodical class, 
about seventy-five years, is 3,293,000,000 
miles from the sun when in aphelion, and 
only 54,770,000 miles when in perihelion. 



Comets 205 

Comets, when near the sun, are greatly- 
affected by the disturbing attraction of 
large planets, and especially of the most 
massive of them all, Jupiter. The effect of 
this disturbance is to change the form of 
their orbits, with the not infrequent result 
that the latter are altered from apparent 
parabolas into unquestionable ellipses, and 
thus the comets concerned are said to be 
"captured," or made prisoners to the sun, 
by the influence of the disturbing planet. 
About twenty small comets are known as 
"Jupiter's Comet Family," because they 
appear to have been "captured" in this way 
by him. A few others are believed to have 
been similarly captured by Saturn, Uranus, 
and Neptune. 

The orbits of comets differ from those of 
the planets in other ways beside their greater 
eccentricity. The planets all move round the 
sun from west to east, but comets move in 
both directions. The orbits of the planets, 
with the exception of some of the asteroids, 
all lie near one common plane, but those of 
comets are inclined at all angles to this plane, 
some of them coming down from the north 
side of the ecliptic and others up from the 
south side. 



206 The Solar System 

A comet consists of two distinct portions : 
first, the head, or nucleus; and, second, the 
tail. The latter only makes its appearance 
when the comet is drawing near the sun, and, 
as a whole, it is always directed away from 
the sun, but usually more or less curved 
backward along the comet's course, as if 
the head tended to run away from it. The 
appearance of a comet's tail at once suggests 
that it is produced by some repulsive force 
emanating from the sun. Recently there 
has been a tendency to explain this on the 
principle of what is known as the pressure 
of light. This demands a brief explanation. 
Light is believed to be a disturbance of the 
universal ether in the form of waves which 
proceed from the luminous body. These 
waves possess a certain mechanical energy 
tending to drive away bodies upon which 
they impinge. The energy is relatively 
slight, and in ordinary circumstances pro- 
duces no perceptible effect, but when the 
body acted upon by the light is extremely 
small the pressure may become so great 
relatively to gravitation as to prevail over 
the latter. To illustrate this, let us recall 
two facts — first, that gravitation acts upon 
the mass, i.e. all the particles of a body 



Comets 207 

throughout its entire volume; and, second, 
that pressure acts only upon the exterior 
surface. Consequently gravitation is pro- 
portional to the volume, while the pressure 
of light is proportional to the surface of 
the body acted upon. Now the mass, or 
volume, of any body varies as the cube of 
its diameter, and the surface only as the 
square. If, then, we have two bodies, one 
of which has twice the diameter of the 
other, the mass of the second will be eight 
times less than that of the first, but the 
surface will be only four times less. If the 
second has only one-third the diameter of 
the first, then its mass will be twenty-seven 
times less, but its surface only nine times 
less. Thus we see that as we diminish the 
size of the body, the mass falls off more 
rapidly than the area of the surface, and 
consequently the pressure gains relatively 
to the gravitation. Experiment has cor- 
roborated the conclusions of mathematics 
on this subject, and has shown that when a 
particle of matter is only about one one- 
hundred-thousandth of an inch in diameter 
the pressure of light upon it becomes greater 
than the force of gravitation, and such a 
particle, situated in open space, would be 



208 The Solar System 

driven away from the sun by the light 
waves. This critical size would vary with 
the density of the matter composing the 
particle, but what we have said will serve to 
convey an idea of its minuteness. 

Now in applying this to comets' tails 
it is only necessary to remark that they are 
composed of either gaseous or dusty particles, 
or both, rising from the nucleus, probably 
under the influence of the heat or the electrical 
action of the sun, and these particles, being 
below the critical size, are driven away from 
the sun, and appear in the form of a tail 
following the comet. It may be added that 
the same principle has been evoked to ex- 
plain the corona of the sun, which may be 
composed of clouds of gas or dust kept in 
suspension by the pressure of light. 

The nuclei of comets contain nearly 
their whole mass. The actual mass of no 
comet is known, but it can in no case be very 
great. Moreover, it is probable that the 
nucleus of a comet does not consist of a 
single body, either solid or liquid, but is 
composed of a large number of separate 
small bodies, like a flock of meteors, crowded 
together and constantly impinging upon one 
another. As the comet approaches the sun 



Comets 209 

the nucleus becomes violently agitated, and 
then the tail begins to make its appearance. 
The possibility exists of an encounter 
between the earth and the head of a comet, 
but no such occurrence is known. Two or 
three times, however, the earth is believed 
to have gone through the tail of a comet, 
the last time in 1910, when Halley's comet 
passed between the earth and the sun, 
but no certain effects have been observed 
from such encounters. The spectroscope 
shows that comets contain various hydro- 
carbons, sodium, nitrogen, magnesium, and 
possibly iron, but we know, as yet, very 
little about their composition. The presence 
of cyanogen gas was reported in Halley's 
comet at its last appearance. We are still 
more ignorant of the origin of comets. We 
do know, however, that they tend to go to 
pieces, especially those which approach very 
close to the sun. The great comet of 1882, 
which almost grazed the sun, was afterward 
seen retreating into space scattered into 
several parts, each provided with a tail. 
In at least one case, several comets have 
been found travelling in the same track, 
an indication that one large original comet 
has been separated into three or four smaller 
14 



210 THe Solar System 

ones. This appears to be true of the comets 
of 1843, 1880, and 1882, — and perhaps the 
comet of 1576 should be added. But the 
most remarkable case of disruption is that of 
Biela's Comet, which first divided into two 
parts in 1846 and then apparently became 
scattered into a swarm of meteors which was 
encountered by the earth in 1872, when it 
passed near the old track of the comet. 
This leads us to our next subject. 

8. Meteors. Everybody must, at some 
time, have beheld the phenomenon known as 
a falling, or shooting, star. A few of these 
objects can be seen darting across the sky 
on almost any clear night in the course of 
an hour or two of watching. Sometimes 
they appear more numerously, and at in- 
tervals they are seen in "showers." They 
are called meteors, and it is believed that 
they are minute solid bodies, perhaps aver- 
aging but a small fraction of an ounce in 
weight, which plunge into the atmosphere 
with velocities varying from twenty to 
thirty or more miles per second, and are 
set afire and consumed by the heat of friction 
developed by their rush through the air. 
Anybody who has seen a bullet melted by 
the heat suddenly developed when it strikes 



Meteors 211 

a steel target has had a graphic illustration 
of the transformation of motion into heat. 
But if we could make the bullet move fast 
enough it would melt in the air, the heat 
being developed by the constant friction. 
The connection of meteors with comets 
is very interesting. In the year 1833, a 
magnificent and imposing display of meteors, 
which, for hours, on the night between the 
13th and 14th of November, filled the sky 
with fire-balls and flaming streaks, astonished 
all beholders and filled many with terror. 
It was found that these meteors travelled 
in an orbit intersecting that of the earth at 
the point where the latter arrived in the 
middle of November, and also that they had 
a period of revolution about the sun of 33^ 
years, and were so far scattered along their 
orbit that they required nearly three years 
to pass the point of intersection with the 
orbit of the earth. Thus it was concluded 
that for three years in succession, in mid- 
November, there should be a display of the 
meteors plunging into the earth's atmos- 
phere. But only in the year when the 
thickest part of the swarm was encountered 
by the earth would the display be very 
imposing. Upon this it was predicted that 



212 THe Solar System 

there would be a recurrence of trie phenomenon 
of 1833 m the y ear 1866. It happened as 
predicted, except that the number of meteors 
was not quite so great as before. In the 
meantime, it had been discovered that these 
meteors followed in the track of a comet 
known as Temple's Comet, and also that 
certain other meteors, which appear e very- 
year in considerable numbers about the 10th 
of August, followed the track of another 
comet called Tut tie's Comet. Then in 
1872 came the display, mentioned in the 
last section, of meteors which were evidently 
the debris of the vanished comet of Biela. 
The inference from so many similar cases 
was irresistible that the meteors must be 
fragments of destroyed or partially destroyed 
comets. Several other cases of identity of 
orbits between meteors and comets have 
been discovered. 

It has been said that the August meteors 
appear every year. The explanation of this 
is that they have, in the course of many 
ages, been scattered around the whole circuit 
of their orbit, so that each year, about 
the 10th of August, when the earth crosses 
their track, some of the meteors are en- 
countered. They are like an endless rail- 



Meteors 213 

road train travelling upon a circular track. 
The November meteors also appear, in small 
numbers, every year, a fact indicating that 
some of them, too, have been scattered all 
around their orbit, although the great mass 
of them is still concentrated in an elongated 
swarm, and a notable display can only occur 
when this swarm is at the crossing simul- 
taneously with the earth. These meteors 
were eagerly awaited in 1899, when it was 
hoped that the splendid displays of 1833 and 
1866 might be repeated, but, unfortunately, 
in the meantime the planets Jupiter and 
Saturn, by their disturbing attractions, had 
so altered the position of the path of the 
meteors in space that the principal swarm 
missed the connection. There are many 
other periodical meteor showers, generally 
less brilliant than those already mentioned, 
and some astronomers think that all of them 
had their origin from comets. 

It is not known that any meteor from any 
of these swarms has ever reached the surface 
of the earth. The meteors appear to be so 
small that they are entirely burnt up before 
they can get through the atmosphere, which 
thus acts as a shield against these little 
missiles from outer space. But there is 



214 THe Solar System 

another class of meteoric bodies, variously 
known as meteorites, aerolites, uranoliths, or 
bolides, which consists of larger masses, 
and these sometimes fall upon the earth, 
after a fiery passage through the air. 
Specimens of them may be seen in many 
museums. They are divided into two 
principal classes, according to their compo- 
sition: first, stony meteorites, which are by 
far the most numerous; and, second, iron 
meteorites, which consist of almost pure iron, 
generally alloyed with a little nickel. The 
stony meteorites, which usually contain 
some compound of iron, consist of a great 
variety of substances, including between 
twenty and thirty different chemical elements. 
Although they resemble in many ways 
minerals of volcanic origin on the earth, 
they also possess certain characteristics by 
which they can be recognised even when 
they have not been seen to fall. 

When a meteorite passes through the air 
it makes a brilliant display of light, and 
frequently bursts asunder, with a tremendous 
noise, scattering its fragments about. The 
largest fragment of a meteorite actually 
seen to fall, weighs about a quarter of a ton. 
Upon striking the ground the meteorite 



Meteors 215 

sometimes penetrates to a depth of several 
feet, and some have been picked up which 
were yet hot on the surface, although very- 
cold within. It is not known that meteorites 
have any connection with comets, and their 
origin can only be conjectured. Among 
the various suggestions that have been made 
the following may be mentioned: (1) that 
they have been shot out of the sun — par- 
ticularly the iron meteorites; (2) that they 
were cast into space by lunar volcanoes 
when the moon was still subject to volcanic 
action; (3) that they are the products 
of explosion in the stars. But some astro- 
nomers are disposed to think that they 
originated in a similar manner to other 
members of the solar system, although it is 
difficult, on this hypothesis, to account for 
their great density. The opinion that the 
iron meteorites have come from the sun, 
or some other star, is enforced by the fact 
that they contain hydrogen, carbon, and 
helium, in forms suggesting that these gases 
were absorbed while the bodies were immersed 
in a hot, dense atmosphere. 



PART IV. 

THE FIXED STARS. 



217 



PART IV. 

THE FIXED STARS. 

i. The Stars. The stars are distant 
suns, varying greatly in remoteness, in 
magnitude, and in condition. Many of 
them are much smaller than our sun, and 
many others are as much larger. They vary, 
likewise in age, or state of development. 
Some are relatively young, others in a 
middle stage, and still others in a condition 
that may be called solar decrepitude. 
These proofs of evolution among the stars, 
the knowledge of which we owe mainly 
to spectroscopic analysis, serve to establish 
more firmly the conclusion, to which the 
simple aspect of the heavens first leads us, 
that the universe is a connected system, 
governed everywhere by similar laws and 
consisting of like materials. 

The number of stars visible to the naked 

eye is about six thousand, but telescopes show 

tens of millions. It is customary to divide the 

stars into classes, called magnitudes, accord- 

219 



220 THe Fixed Stars 

ing to their apparent brightness. By a 
system of photometry, or light-measurement, 
they are grouped into stars of the first, 
second, third, etc., magnitude. With the 
naked eye no stars fainter than the sixth 
magnitude are visible, but very powerful 
telescopes may show them down to the 
eighteenth magnitude. Each magnitude is 
about two and a half times brighter than the 
next magnitude below in the scale. A first- 
magnitude star is about one hundred times 
brighter than one of the sixth magnitude. 
But, in reality, the variation of brightness 
is gradual, and for very accurate estimates 
fractions of a magnitude have to be employed. 
There are about twenty first-magnitude stars, 
but they are not all of equal brightness. 
A more accurate photometry assumes a 
zero magnitude, very nearly, represented 
by the star Arcturus, and makes the ratio 
2.512. Thus a star, nearly represented by 
Aldebaran or Altair, which is 2.512 times 
fainter than the zero magnitude, is of the 
first magnitude, and a star, nearly repre- 
sented by the North Star, which is 2.512 
times fainter than the first magnitude, is of 
the second magnitude. Counting in the 
other direction, a star, like Sirius, which is 



The Stars 221 

brighter than the zero magnitude, is said to 
be of a negative magnitude. The magnitude 
of Sirius is — 1.6. There is only one other 
star of negative magnitude, Canopus, whose 
magnitude is — 0.9. But for ordinary pur- 
poses one need not trouble himself with these 
refinements. 

The stars are divided into five principal 
types, according to their spectra. These 
are: 

I. White stars, having a bluish tinge, in 
which the spectrum is characterised by broad 
dark bands, due apparently to an extensive 
atmosphere of hydrogen, while there are 
but few lines indicating the presence of 
metallic vapours. About half the stars 
whose spectra have been studied belong to 
Type I. 

II. Yellowish- white stars, resembling the 
sun in having their spectra crossed with a 
great number of lines produced by metallic 
vapours, while the hydrogen lines are less 
conspicuous. These are often called solar 
stars, and they, too, are very numerous. 

III. Orange and slightly reddish stars, 
whose spectra contain mostly broad bands 
instead of narrow lines, the bands being 
situated toward the blue end of the spectrum, 



222 THe Fixed Stars 

whence the prevailing colour, since the blue 
light is thus cut off. Only a few hundred 
of these stars are known, but they include 
most of the well-known variable stars. 

IV. Small deep-red stars having dark 
bands absorbing the light of the red end of 
the spectrum. Less than a hundred of 
these stars are known. 

V. Stars whose spectra are characterised 
by bright instead of dark lines, although 
they also show dark bands. The bright 
lines indicate that the atmospheric vapours 
producing them are at a higher temperature 
than the body of the star. Stars of this 
type are sometimes called Wolf-Rayet stars 
and they are few in number. 

Various modifications of these main types 
exist, but we cannot here enter into an 
account of them. In a general way, al- 
though there are exceptions depending upon 
the precise nature of each spectrum, the 
white stars are thought to be younger than 
the yellowish ones, and the red stars older. 

In speaking of the "size" of the stars we 
really mean their luminosity, or the amount 
of light radiated from them. When a star 
is said to be a thousand times greater than 
the sun, the meaning is that the amount of 



The Stars 223 

light that it gives would, if both were viewed 
from the same distance, be equal to a 
thousand times the amount given by the 
sun. We have no direct knowledge of the 
actual size of the stars as globes, because 
the most powerful telescope is unable to 
reveal the real disk of a star. In comparing 
the luminosity of a star with that of the sun 
its distance must be taken into account. 
Most of the stars are so far away that we 
really know nothing of their distances, but 
there are fifty or more which lie within a 
distance not too great to enable us to obtain 
an approximate idea of what it is. The 
nearest star in the northern sky is so far 
from being the brightest that it can barely 
be seen with the naked eye. It must be 
very much less luminous than the sun. On 
the other hand, some very bright stars lie 
at a distance so immense that it can hardly 
be estimated, and they must exceed the sun 
in luminosity hundreds and even thousands 
of times. 

The question of the distance of the stars 
has already been treated in the section on 
Parallax. In employing our knowledge of 
star distances for the purpose of comparing 
their luminosity with that of the sun, we 



224 THe Fixed Stars 

must first ascertain, as accurately as possible, 
the actual amount of light that the star 
sends to the earth as compared with the 
actual amount of light that the sun sends. 
The star Arcturus gives to our eyes about 
one forty-billionth as much light as the sun 
does. Knowing this, we must remember 
that the intensity of light varies, like gravi- 
tation, inversely as the square of the distance. 
Thus, if the sun were twice as far away as it 
is, the amount of its light received on the 
earth would be reduced to one fourth, and 
if its distance were increased three times, the 
amount would be reduced to one ninth. 
If the sun were 200,000 times as far away, 
its light would be reduced to one forty-bil- 
lionth, or the same as that of Arcturus. 
At this point the actual distance of Arcturus 
enters into the calculation. If that distance 
were 200,000 times the sun's distance, we 
should have to conclude that Arcturus 
was exactly equal to the sun in luminosity, 
since the sun, if removed to the same distance, 
would give us the same amount of light. 
But, in fact, we find that the distance of 
Arcturus, instead of being 200,000 times that 
of the sun, is about 10,000,000 times. In 
other words, it is fifty times as far away as 



The Stars 225 

the sun would have to be in order that it 
should appear to our eyes no brighter than 
Arcturus. From this it follows that the 
real luminosity of Arcturus must be the 
square of 50, or 2500, times that of the sun. 
In the same manner we find that Sirius, 
which to the eye appears to be the brightest 
star in the sky (much brighter than Arcturus 
because much nearer), is about thirty times 
as luminous as the sun. 

Many of the stars are changeable in 
brightness, and those in which the changes 
occur to a notable extent, and periodically, 
are known as variable stars. It is probable 
that all the stars, including the sun, are 
variable to a slight degree. Among the 
most remarkable variables are Mir a, or 
Omicron Ceti, in the constellation Cetus, 
which in the course of about 331 days rises 
from the ninth to the third magnitude and 
then falls back again (the maxima of bright- 
ness are irregular) ; and Algol, or Beta Persei, 
in the constellation Perseus, which, in a 
period of 2 days, 20 hours, 49 minutes, 
changes from the third to the second magni- 
tude and back again. In the case of Mira 
the cause of the changes is believed to lie 
in the star itself, and they may be connected 



226 THe Fixed Stars 

with its gradual extinction. The majority 
of the variable stars belong to this class. 
As to Algol, the variability is apparently 
due to a huge dark body circling close around 
the star with great speed, and periodically 
producing partial eclipses of its light. There 
are a few other stars with short periods of 
variability which belong to the class of Algol. 
When examined with telescopes many of 
the stars are found to be double, triple, or 
multiple. Often this arises simply from the 
fact that two or more happen to lie in nearly 
the same line of sight from the earth, but 
in many other cases it is found that there 
is a real connection, and that the stars con- 
cerned revolve, under the influence of their 
mutual gravitation, round a common centre 
of force. When two stars are thus connected 
they are called a binary. The periods of 
revolution range from fifty to several 
hundred years. Among the most celebrated 
binary stars are Alpha Centauri, in the 
southern hemisphere, the nearest known 
star to the solar system, whose components 
revolve in a period of about eighty years; 
Gamma Virginis, in the constellation Virgo, 
period about one hundred and seventy years ; 
and Sirius, period about fifty-three years. 



Trie Stars <227 

In the case of Sirius, one of the components, 
although perhaps half as massive as its com- 
panion, is ten thousand times less bright. 

There is another class of binary stars, 
in which one of the companions is invisible, 
its presence being indicated by the effects 
of its gravitational pull upon the other. 
Algol may be regarded as an example of 
this kind of stellar association. But there 
are stars of this class, where the companion 
causes no eclipses, either because it is not 
dark, or because it never passes over the 
other, as seen from the earth, but where its 
existence is proved, in a very interesting way, 
by the spectroscope. In these stars, called 
spectroscopic binaries, two bright components 
are so close together that no telescope is 
able to make them separately visible, but 
when their plane of revolution lies nearly 
in our line of sight the lines in their combined 
spectrum are seen periodically split asunder. 
To understand this, we must recall the 
principles underlying spectroscopic analysis 
and add something to what was said before 
on that subject. * 

Light consists of waves in the ether of 
different lengths and making upon the eye 
different impressions of colour according 



228 THe Fixed Stars 

to the length of the waves. The longest 
waves are at the red end of the spectrum 
and the shortest at the blue, or violet, end. 
But since they all move onward with the 
same speed, it is clear that the short blue 
waves must fall in quicker succession on 
the retina of the eye than the long red waves. 
Now suppose that the source of light from 
which the waves come is approaching very 
swiftly; it is easy to see that all the waves 
will strike the eye with greater rapidity, 
and that the whole spectrum will be shifted 
toward the blue, or short-wave, end. The 
Fraunhofer lines will share in this shifting 
of position. Next suppose that the source 
of the light is retreating from the eye. The 
same effect will occur in a reversed sense, 
for now there will be a general shift toward 
the red end of the spectrum. A sufficiently 
clear illustration, by analogy, is furnished 
by the waves of sound. We know that 
low-pitched sounds are produced by long 
waves, and high-pitched ones by short waves; 
then if the source of the sound, such as a 
locomotive whistle, rapidly approaches the 
ear the waves are crowded together, or 
shifted as a whole toward the short end of 
the gamut, whereupon the sound rises to a 



THe Stars 229 

shrill scream. If, on the contrary, the 
source of sound is retreating, the shift is in 
the other direction, and the sound drops 
to a lower pitch. 

This is precisely what happens in the 
spectrum of a star which is either approach- 
ing or receding from the eye. If it is ap- 
proaching, the Fraunhofer lines are seen 
shifted out of their normal position toward 
the blue, and if it is receding they are shifted 
toward the red. The amount of shifting 
will depend upon the speed of the star's 
motion. If that motion is across the line 
of sight there will be no shifting, because 
then the source of light is neither approaching 
nor receding. Now take the case of a binary 
star whose components are too close to be 
separated by a telescope. If they happen 
to be revolving round their common centre 
in a plane nearly coinciding with the line 
of sight from the earth, one of them must 
be approaching the eye at the same time that 
the other is receding from it, and the con- 
sequence is that the spectral lines of the first 
will be shifted toward the blue, while those 
of the second are shifted toward the red. 
The colours of the two intermingled spectra 
blend into each other too gradually to 



230 TKe Fixed Stars 

enable this effect to be detected by their 
means, but the Fratmhofer lines are sharply- 
defined, and in them the shift is clearly seen ; 
and since there is a simultaneous shifting 
in opposite directions the lines appear split. 
But when the two stars are in that part 
of their orbit where their common motion is 
across the line of sight the lines close up 
again, because then there is no shift. This 
phenomenon is beautifully exhibited by one 
of the first spectroscopic binaries to be dis- 
covered, Beta Aurigse. In 1889, Prof. E. 
C. Pickering noticed that the spectral lines 
of this star appeared split every second night, 
from which he inferred that it consisted of 
two stars revolving round a common centre 
in a period of four days. 

This spectroscopic method has been applied 
to determine the speed with which certain 
single stars are approaching or receding 
from the solar system. It has also served 
to show, what we have before remarked, 
that the inner parts of Saturn's rings travel 
faster than the outer parts. Moreover, it 
has been used in measuring the rate of the 
sun's rotation on its axis, for it is plain that 
one edge of the sun approaches us while 
the opposite edge is receding. Even the 



The Stars 231 

effect of the rotation of Jupiter has been 
revealed in this way, and the same method 
will probably settle the question whether 
Venus rotates rapidly, or keeps the same face 
always toward the sun. 

Not only do many stars revolve in orbits 
about near-by companions, but all the stars, 
without exception, are independently in 
motion. They appear to be travelling 
through space in many different directions, 
each following its own chosen way without 
regard to the others, and each moving at its 
own gait. These movements of the stars 
are called proper motions. The direction 
of the sun's proper motion is, roughly speak- 
ing, northward, and it travels at the rate of 
twelve or fourteen miles per second, carry- 
ing the earth and the other planets along 
with it. Some stars have a much greater 
speed than the sun, and some a less speed. 
As we have said, these motions are in many 
different directions, and no attempt to 
discover any common law underlying them 
has been entirely successful, although it has 
been found that in some parts of the sky 
a certain number of stars appear to be 
travelling along nearly parallel paths, like 
flocks of migrating birds. In recent years 



232 TKe Fixed Stars 

some indications have been found of the 
possible existence of two great general currents 
of movement, almost directly opposed to 
each other, part of the stars following one 
current and part the other. But no indica- 
tion has been discovered of the existence 
of any common centre of motion. Several 
relatively near-by stars appear to be moving 
in the same direction as the sun. Stars 
that are closely grouped together, like the 
cluster of the Pleiades, seem to share a 
common motion of translation through 
space. We have already remarked that 
when stars are found to be moving toward 
or away from the sun, spectroscopic obser- 
vation of the shifting of their lines gives a 
means of calculating their velocity. In 
other cases, the velocity across the line of 
sight can be calculated if we know the 
distance of the stars concerned. One in- 
teresting result of the fact that the earth 
goes along with the sun in its flight is that 
the orbit of the earth cannot be a closed 
curve, but must have the form of a spiral 
in space. In consequence of this we are 
continually advancing, at the rate of at 
least 400,000,000 miles per year, toward the 
northern quarter of the sky. The path 



The Stars 233 

pursued by the sun appears to be straight, 
although it may, in fact, be a curve so large 
that we are unable in the course of a lifetime, 
or many lifetimes, to detect its departure 
from a direct line. At any rate we know 
that, as the earth accompanies the sun, 
we are continually moving into new regions 
of space. 

It has been stated that many millions of 
stars are visible with telescopes — perhaps 
a hundred millions, or even more. The 
great majority of these are found in a broad 
irregular band, extending entirely round the 
sky, and called the Milky Way, or the 
Galaxy. To the naked eye the Milky Way 
appears as a softly shining baldric encircling 
the heavens, but the telescope shows that 
it consists of multitudes of faint stars, 
whose minuteness is probably mainly due 
to the immensity of their distance, although 
it may be partly a result of their relative 
lack of actual size, or luminosity. In many 
parts of the Milky Way the stars appear 
so crowded that they present the appearance 
of sparkling clouds. The photographs of 
these aggregations of stars in the Milky 
Way, made by Barnard, are marvellous 
beyond description. In the Milky Way, and 



234 THe Fixed Stars 

sometimes outside it, there exist globular 
star-clusters, in which the stars seem so 
crowded toward the centre that it is im- 
possible to separate them with a telescope, 
and the effect is that of a glistering ball 
made up of thousands of silvery particles, 
like a heap of microscopic thermometer 
bulbs in the sunshine. A famous cluster 
of this kind is found in the constellation 
Hercules. 

The Milky Way evidently has the form 
of a vast wreath, made up of many interlaced 
branches, some of which extend considerably 
beyond its mean borders. Within, this 
starry wreath space is relatively empty of 
stars, although some thousands do exist 
there, of which the sun is one. We are at 
present situated not very far from the centre 
of the opening within the ring or wreath, but 
the proper motion of the sun is carrying us 
across this comparatively open space, and in 
the course of time, if the direction of our 
motion does not change, we shall arrive at 
a point not far from its northern border. 
The Milky Way probably indicates the 
general plan on which the visible universe 
is constructed, or what has been called the 
architecture of the heavens, but we still 



THe Stars 235 

know too little of this plan to be able to say- 
exactly what it is. 

The number of stars in existence at any- 
time varies to a slight degree, for occasion- 
ally a star disappears, or a new one makes 
its appearance. These, however, are rare 
phenomena, and new stars usually disappear 
or fade away after a short time, for which 
reason they are often called temporary stars. 
The greatest of these phenomena ever beheld 
was Tycho Brahe's star, which suddenly 
burst into view in the constellation Cassiopeia 
in 1572, and disappeared after a couple of 
years, although at first it was the brightest 
star in the heavens. Another temporary 
star, nearly as brilliant, appeared in the 
constellation Perseus, in 1901, and this, as 
it faded, gradually turned into a nebula, 
or a star surrounded by a nebula. It is 
generally thought that outbursts of this 
kind are caused by the collision of two or 
more massive bodies, which were invisible 
before their disastrous encounter in space. 
The heat developed by such a collision would 
be sufficient to vaporise them, and thus to 
produce the appearance of a new blazing 
star. It is possible that space contains an 
enormous number of great obscure bodies, 



236 THe Fixed Stars 

— extinguished suns, perhaps — which are 
moving in all directions as rapidly as the 
visible stars. 

2. The Nebulae. These objects, which get 
their name from their cloud- like appearance, 
are among the most puzzling phenomena of 
the heavens, although they seem to suggest 
a means of explaining the origin of stars. 
Many thousands of nebulae are known, but 
there are only two or three bright enough to 
be visible to the naked eye. One of these 
is in the " sword" of the imaginary giant 
figure marking the constellation Orion, and 
another is in the constellation Andromeda. 
They look to the unaided eye like misty 
specks, and require considerable attention 
to be seen at all. But in telescopes their 
appearance is marvellous. The Orion nebula 
is a broad, irregular cloud, with many brighter 
points, and a considerable number of stars 
intermingled with it, while the Andromeda 
nebula has a long spindle shape, with a 
brighter spot in the centre. It is covered 
and surrounded with multitudes of faint 
stars. It was only after astronomical pho- 
tography had been perfected that the real 
shapes of the nebulae were clearly revealed. 
Thousands of nebulae have been discovered 



THe Nefcmlae 237 

by photography, which are barely if at 
all visible to the eye, even when aided 
by powerful telescopes. This arises from 
the fact that the sensitive photographic 
plate accumulates the impression that the 
light makes upon it, showing more and more 
the longer it is exposed. Plates placed in 
the focus of telescopes, arranged to utilise 
specially the "photographic rays," are often 
exposed for many hours on end in order to 
picture faint nebulae and faint stars, so 
that they reveal things that the eye, which 
sees all it can see at a glance, is unable 
to perceive. 

Nebulae are generally divided into two 
classes — the ' ' white ' ' nebulae and the ' ' green ' ' 
nebulae. The first, of which the Andromeda 
nebula is a striking example, give a con- 
tinuous spectrum without dark lines, as if 
they consisted either of gas under high 
pressure, or of something in a solid or liquid 
state. The second, conspicuously repre- 
sented by the Orion nebula, give a spectrum 
consisting of a few bright lines, characteristic 
of such gases as hydrogen and helium, 
together with other substances not yet 
recognised. But there is no continuous 
spectrum like that shown by the white nebulae, 



238 TKe Fixed Stars 

from which it is inferred that the green 
nebulae, at least, are wholly gaseous in 
their constitution. The precise constitution 
of the white nebulae remains to be determined. 
It is only in relatively recent years that 
the fact has become known that the majority 
of nebulae have a spiral form. There is 
almost invariably a central condensed mass 
from which great spiral arms wind away on 
all sides, giving to many of them the ap- 
pearance of spinning pin- wheels, flinging 
off streams of fire and sparks on all sides. 
The spirals look as if they were gaseous, 
but along and in them are arrayed many 
condensed knots, and frequently curving 
rows of faint stars are seen apparently in 
continuation of the nebulous spirals. The 
suggestion conveyed is that the stars have 
been formed by condensation from the spirals. 
These nebulae generally give the spectra 
of the white class, but there are also some- 
times seen bright lines due to glowing gases. 
The Andromeda nebula is sometimes de- 
scribed as spiral, but its aspect is rather 
that of a great central mass surrounded with 
immense elliptical rings, some of which have 
broken up and are condensing into separate 
masses. The Orion nebula is a chaotic 



The Nefcmlae 239 

cloud, filled with partial vacancies and ribbed 
with many curving, wave-like forms. 

There are other nebulae which have the 
form of elliptical rings, occasionally with one 
or more stars near the centre. A famous 
example of this kind is found in the constel- 
lation Lyra. Still others have been compared 
in shape to the planet Saturn with its rings, 
and some are altogether bizarre in form, 
occasionally looking like glowing tresses 
floating among the stars. 

The apparent association of nebulae with 
stars led to the so-called nebular hypothesis, 
according to which stars are formed, as 
already suggested, by the condensation of 
nebulous matter. In the celebrated form 
which Laplace gave to this hypothesis, it was 
concerned specially with the origin of our solar 
system. He assumed that the sun was once 
enormously expanded, in a nebulous state, or 
surrounded with a nebulous cloud, and that 
as it contracted rings were left off around 
the periphery of the vast rotating mass. 
These rings subsequently breaking and con- 
densing into globes, were supposed to have 
given rise to the planets. It is still believed 
that the sun and the other stars may have 
originated from the condensation of nebulae, 



240 THe Fixed Stars 

but many objections have been found to the 
form in which Laplace put his hypothesis, 
and the discovery of the spiral nebulae has 
led to other conjectures concerning the way 
in which the transformation is brought 
about. But we have not here the space to 
enter into this discussion, although it is of 
fascinating interest. 

A word more should be said about the use 
of photography in astronomy. It is hardly 
going too far to aver that the photographic 
plate has taken the place of the human retina 
in recording celestial phenomena, especially 
among the stars and nebulae. Not only 
are the forms of such objects now exclusively 
recorded by photography, but the spectra 
of all kinds of celestial objects — sun, stars, 
nebulae, etc. — are photographed and afterward 
studied at leisure. In this way many of the 
most important discoveries of recent years 
have been made, including those of variable 
stars and new stars. Photographic charts 
of the heavens exist, and by comparing these 
with others made later, changes which would 
escape the eye can be detected. Comets 
are sometimes, and new asteroids almost 
invariably, discovered by photography. The 
changes in the spectra of comets and new 



THe Constellations 241 

stars are thus recorded with an accuracy that 
would be otherwise unattainable. Photo- 
graphs of the moon excel in accuracy all 
that can be done by manual drawing, and 
while photographs of the planets still fail 
to show many of the fine details visible with 
telescopes, continual improvements are being 
made. Many of the great telescopes now 
in use or in course of construction are 
intended specially for photographic work. 

3. The Constellations. The division of 
the stars into constellations constitutes the 
uranography or the " geography of the 
heavens." The majority of the constellations 
are very ancient, and their precise origin is 
unknown, but those which are invisible 
from the northern hemisphere have all been 
named since the great exploring expeditions 
to the south seas. There are more than 
sixty constellations now generally recognised. 
Twelve of these belong to the zodiac, and 
bear the same names as the zodiacal signs, 
although the precession of the equinoxes 
has drifted them out of their original relation 
to the signs. Many of the constellations 
are memorials of prehistoric myths, and a 
large number are connected with the story 
of the Argonautic expedition and with 
16 



242 THe Fixed Stars 

other famous Greek legends. Thus the 
constellations form a pictorial scroll of 
legendary history and mythology, and possess 
a deep interest independent of the science 
of astronomy. For their history and for 
the legends connected with them, the reader 
who desires a not too detailed resume, may 
consult Astronomy with the Naked Eye, and 
for guidance in finding the constellations, 
Astronomy with an Opera-glass, or Round 
the Year with the Stars. The quickest way to 
learn the constellations is to engage the aid 
of some one who knows them already, and 
can point them out in the sky. The next 
best way is to use star charts, or a star-finder 
or planisphere. 

A considerable number of the brighter and 
more important stars are known by indi- 
vidual names, such as Sirius, Canopus, 
Achernar, Arcturus, Vega, Rigel, Betelguese, 
Procyon, Spica, Aldebaran, Regulus, Altair, 
and Fomalhaut. Astronomers usually desig- 
nate the principal stars of each constella- 
tion by the letters of the Greek alphabet, 
a, p, y, etc., the brightest star in the con- 
stellation bearing the name of the first 
letter, the next brightest that of the second 
letter, and so on. 



THe Constellations 243 

The constellations are very irregular in 
outline, and their borders are only fixed 
with sufficient definiteness to avoid the 
inclusion of stars catalogued as belonging 
to one, within the limits of another. In all 
cases the names come from some fancied 
resemblance of the figures formed by the 
principal stars of the constellation to a man, 
woman, animal, or other object. In only 
a few cases are these resemblances very 
striking. 

The most useful constellations for the 
beginner are those surrounding the north 
celestial pole, and we give a little circular 
chart showing their characteristic stars. 
The names of the months running round the 
circle indicate the times of the year when 
these constellations are to be seen on or 
near the meridian in the north. Turn the 
chart so that the particular month is at the 
bottom, and suppose yourself to be facing 
northward. The hour when the observation 
is supposed to be made is, in every case, 
about 9 o'clock in the evening, and the date 
is about the first of the month. The top of 
the chart represents the sky a little below 
the zenith in the north, and the bottom 
represents the horizon in the north. 



244 



THe Fixed Stars 



The apparent yearly revolution of the 
heavens, resulting from the motion of the 
earth in its orbit, causes the constellations 
to move westward in a circle round the pole, 



03J 




£p r< , aU 

Fig. 18. The North Circumpolar Stars. 

at the rate of about 30 ° per month. But the 
daily rotation of the earth on its axis causes 
a similar westward motion of the heavens, 
at the rate of about 30 ° for every two hours. 
From this it results that on the same night, 



THe Constellations 



245 



after an interval of two hours, you will see the 
constellations occupying the place that they 
will have, at the original hour of observation, 
one month later. Thus, if you observe their 



st^T^* t ~r^>^ 




A ' \ 


/ • • \ • 




>sNI \ f*. JV* *n. 




/ <T6« •■ iVi \ 


7 * «>* •'• * \ 


/ ■ ^V •,» «\ 


\ //• S* y> \ O \ 


/ «• \ %<ms\' 


y /^ • ° '•• % \ 

/ ^-tr ! % \ 


.*» \ ; %; *^js^g3^r-'"* ,•••"' '"••< 1 


\ ^ * -jy^ / 






•'•■• \ CaA, • t-'7 


\ ~*^^ • .'/ 


\ **. A*\^ / 


V^o . ./ 


\ "*' •' ^^\ / 


\ • • / CAMEL( 


\ --^' 7 


>v / • * •'• 


\\ &y 


>o^ ' 1 


1 \.\i V/ 


^~->~^ 


AURIGA A^-*^"'^ 


i<Yg. 19. Key to Nor 


^ Circumpolar Stars. 



positions at 9 p.m. on the first of January, and 
then turn the chart so as to bring February at 
the bottom, you will see the constellations 
around the north pole of the heavens placed as 
they will be at 1 1 p.m. on the first of January. 



246 THe Fixed Stars 

Only the conspicuous stars have been 
represented in the chart, just enough being 
included to enable the learner to recognise 
the constellations by their characteristic 
star groups, from which they have received 
their names. The chart extends to a dis- 
tance of 40 from the pole, so that, for 
observers situated in the mean latitude of 
the United States, none of the constellations 
represented ever descends below the horizon, 
those that are at the border of the chart 
just skimming the horizon when they are 
below the pole. 

On the key to the chart the Greek-letter 
names of the principal stars have been at- 
tached, but some of them have other names 
which are more picturesque. These are 
as follows: In Ursa Major (the Great Bear, 
which includes the Great Dipper), a _is^ 
called Dubhe, $ Merak , 7 Phaed, 8 Megrez, 
£ Alioth, £ Mizar, an3 "q Benetnash. The 
little star close by Mizar is Alcor. In 
Cassiopeia, a is called Schedar, (J Caph, 
and 5 Ruchbar. In Ursa Minor, the Little 
Bear, <x is called Polaris, or the North Star, 
and § Kochab. In Draco, a is called Thuban, 
and t Eltanin. In Cepheus, a is called 
Alderamin, and § Alfirk. These names are 



THe Constellations 247 

nearly all of Arabic origin. It will be ob- 
served that I^lerak and Dubhe are the 
famous "Pointers, which serveTto indicate 
the position of the North Star, while Thuban 
is the "star of the pyramid," before men- 
tioned. The north celestial pole is situated 
almost exactly on a straight line drawn from 
Mizar through the North Star to Ruchbar, 
and a little more than a degree from the 
North Star in the direction of Ruchbar. 
This furnishes a ready means for ascertaining 
the position of the meridian. For instance, 
about the middle of October, Mizar is very 
close to the meridian below the pole, and 
Ruchbar equally close to it above the pole, 
and then, since the North Star is in line with 
these two, it also must be practically on 
the meridian, and its direction indicates 
very nearly true north. The same method 
is applicable whenever, at any other time 
of the year or of the night, Mizar and 
Ruchbar are observed to lie upon a vertical 
line, no matter which is above and which 
below. It is also possible to make a very 
good guess at the time of night by knowing 
the varying position of the line joining 
these stars. 

The star Caph is an important landmark 



248 Trie Fixed Stars 

because it lies almost on the great circle 
of the equinoctial colure, which passes through 
the vernal and autumnal equinoxes. 

On the key, the location of the North Pole 
of the Ecliptic is shown, and the greater 
part of the circle described by the north 
celestial pole in the period of 25,800 years. 

While the reader who wishes to pursue 
the study of the constellations in detail 
must be referred to some of the works 
before mentioned, or others of like character, 
it is possible here to aid him in making a 
preliminary acquaintance with other con- 
stellations beside those included in our little 
chart, by taking each of the months in turn, 
and describing the constellations which he 
will see on or near the meridian south of the 
border of the chart at the same time that 
the polar constellations corresponding to 
the month selected are on or near the me- 
ridian in the north. Thus, at 9 p.m. about 
the first of January, the constellation Per- 
seus, lying in a rich part of the Milky Way, 
is nearly overhead and directly south of the 
North Star. This constellation is marked by 
a curved row of stars, the brightest of which, 
of the second magnitude, is Algenib, or 2 
Persei. A few degrees south-west of Algenib 



THe Constellations 249 

is the wonderful variable Algol. East of 
Perseus is seen the very brilliant white star 
Capella in the constellation Auriga. This 
is one of the brightest stars in the sky. 
Almost directly south of Perseus, the eye 
will be caught by the glimmering cluster 
of the Pleiades in the constellation Taurus. 
A short distance south-east of the Pleiades 
is the group of the Hyades in Taurus, shaped 
like the letter V, with the beautiful reddish 
star Aldebaran in the upper end of the 
southern branch of the letter. The ecliptic 
runs between the Pleiades and the Hyades. 
Still lower in the south will be seen a part 
of the long-winding constellation Eridanus, 
the River Po. Its stars are not bright but 
they appear in significant rows and streams. 
About the first of February the constel- 
lation Auriga is on the meridian not far from 
overhead, Capella lying toward the west. 
Directly under Auriga, two rather conspicu- 
ous stars mark the tips of the horns of 
Taurus, imagined as a gigantic bull, and 
south of these, with its centre on the equa- 
tor, scintillates the magnificent constellation 
Orion, the most splendid in all the sky, 
with two great first-magnitude stars, one, 
in the shoulder of the imaginary giant, 



250 THe Fixed Stars 

of an orange hue, called Betelguese, and the 
other in the foot, of a blue- white radiance, 
called Rigel. Between these is stretched 
the straight line of the "belt," consisting 
of three beautiful second-magnitude stars, 
about a degree and a half apart. Their 
names, beginning with the western one, 
are Mintaka, Alnilam, and Alnitah. Di- 
rectly under the belt, in the midst of a short 
row of faint stars called the "sword," is 
the great Orion nebula. It will be observed 
that the three stars of the belt point, though 
not exactly, toward the brightest of all 
stars, Sirius, in the constellation Canis Major, 
the Great Dog, which is seen advancing from 
the east. Under Orion is a little constellation 
named Lepus, the Hare. 

The first of March the region overhead is 
occupied by the very faint constellation 
Lynx. South of it, and astride the ecliptic, 
appear the constellations Gemini, the Twins, 
and Cancer, the Crab. These, like Taurus, 
belong to the zodiac. The Twins are west- 
ward fron Cancer, and are marked by two 
nearly equal stars, about five degrees apart. 
The more westerly and northerly one is 
Castor and the other is Pollux. Cancer 
is marked by a small cluster of faint stars 



THe Constellations 251 

called Praesepe, the Manger (also sometimes 
the Beehive). Directly south of the Twins, 
is the bright lone star Procyon, in the con- 
stellation Canis Minor, the Little Dog. 
Sirius and the other stars of Canis Major, 
which make a striking figure, are seen south- 
west of Procyon. 

The first of April the zodiac constellation 
Leo is near the meridian, recognisable by 
a sickle-shaped figure marking the head and 
breast of the imaginary Lion. The bright 
star at the end of the handle of the sickle 
is Regulus. Above Leo, between it and the 
Great Dipper, appears a group of stars 
belonging to the small constellation Leo 
Minor, the Little Lion. Farther south is 
a winding ribbon of stars indicating the 
constellation Hydra, the Water Serpent. 
Its chief star, Alphard, of a slightly reddish 
tint, is seen west of the meridian and a few 
degrees south of the equator. 

At the beginning of May, when the Great 
Dipper is nearly overhead, the small con- 
stellation Canes Venatici, the Hunting Dogs, 
is seen directly under the handle of the 
Dipper, and south of that a cobwebby spot, 
consisting of minute stars, indicates the 
position of the constellation Coma Berenices, 



252 The Fixed Stars 

Berenice's Hair. Still farther south, where 
the ecliptic and the equator cross, at the 
autumnal equinox, is the large constellation 
Virgo, the Virgin, also one of the zodiacal 
band. Its chief star Spica, a pure white 
gem, is seen some 20° east of the meridian. 
Below and westward from Virgo, and south 
of the equator, are the constellations Crater, 
the Cup, and Corvus, the Crow. The stars 
of Hydra continue to run eastward below 
these constellations. The westernmost, Cra- 
ter, consists of small stars forming a rude 
semicircle open toward the east, while Corvus, 
which possesses brighter stars, has the form 
of a quadrilateral. 

The first of June the great golden star 
Arcturus, whose position may be found by 
running the eye along the curve of the handle 
of the Great Dipper, and continuing onward 
a distance equal to the whole length of the 
Dipper, is seen approaching the meridian 
from the east and high overhead. This 
superb star is the leader of the constellation 
Bootes, the Bear-Driver. Spica in Virgo is 
now a little west of the meridian. 

The first of July, when the centre of Draco 
is on the meridian north of the zenith, the 
exquisite circlet of stars called Corona 



THe Constellations 253 

Borealis, the Northern Crown, is nearly 
overhead. A short distance north-east of 
it appears a double-quadrilateral figure, 
marking out the constellation Hercules, 
while directly south of the Crown a crooked 
line of stars trending eastward indicates the 
constellation Serpens, the Serpent. South- 
west of Serpens, two widely separated but 
nearly equal stars of the second magnitude 
distinguish the zodiacal constellation Libra, 
the Balance; while lower down toward 
the south-east appears the brilliant red star 
Antares, in the constellation Scorpio, likewise 
belonging to the zodiac. 

On the first of August the head of Draco 
is on the meridian near the zenith, and south 
of it is seen Hercules, toward the west, and 
the exceedingly brilliant star Vega, in the 
constellation Lyra, the Lyre, toward the 
east. Vega, or Alpha Lyrse, has few rivals 
for beauty. Its light has a decided bluish- 
white tone, which is greatly accentuated 
when it is viewed with a telescope. South 
of Hercules two or three rows of rather 
large, widely separated stars mark the 
constellation Ophiuchus, the Serpent- Bearer. 
This extends across the equator. Below it, 
in a rich part of the Milky Way, is Scorpio, 



254 THe Fixed Stars 

whose winding line, beginning with Antares 
west of the meridian, terminates a consider- 
able distance east of the meridian in a pair of 
stars representing the uplifted sting of the 
imaginary monster. 

The first of September the Milky Way runs 
directly overhead, and in the midst of it 
shines the large and striking figure called 
the Northern Cross, in the constellation 
Cygnus, the Swan. The bright star at the 
head of the Cross is named Denib. Below 
the Cross and in the eastern edge of the 
Milky Way is the constellation Aquila, the 
Eagle, marked by a bright star, Altair, 
with a smaller one on each side and not 
far away. Low in the south, a little west 
of the meridian and partly immersed in 
the brightest portion of the Milky Way, 
is the zodiacal constellation Sagittarius, 
the Archer. It is distinguished by a group 
of stars several of which form the figure 
of the upturned bowl of a dipper, some- 
times called the Milk Dipper. East of 
Cygnus and Aquila a diamond-shaped figure 
marks the small constellation Delphinus, 
the Dolphin. 

At the opening of October, when Denib 
is near the meridian, the sky directly in the 



THe Constellations 255 

south is not very brilliant. Low down, 
south of the equator, is seen the zodiacal 
constellation Capricornus, the Goat, with 
a noticeable pair of stars in the head of the 
imaginary animal. 

On the first of November, when Cassiopeia 
is approaching the meridian overhead, the 
Great Square, in the constellation Pegasus, 
is on the meridian south of the zenith, 
while south-west of Pegasus the zodiacal 
constellation Aquarius, the Water-Bearer, 
appears on the ecliptic. A curious scrawling 
Y-shaped figure in the upper part of Aquarius 
serves as a mark to identify the constellation. 
Thirty degrees south of this shines the bright 
star Fomalhaut, in the constellation Piscis 
Australis, the Southern Fish. The two 
stars forming the eastern side of the Great 
Square of Pegasus are interesting because, 
like Caph in Cassiopeia, they lie close to the 
line of the equinoctial colure. The northern 
one is called Alpheratz and the southern 
Gamma Pegasi. Alpheratz is a star claimed 
by two constellations, since it not only marks 
one corner of the square of Pegasus, but 
it also serves to indicate the head of the 
maiden in the celebrated constellation of 
Andromeda. 



256 THe Fixed Stars 

The first of December, Andromeda is seen 
nearly overhead, south of Cassiopeia. The 
constellation is marked by a row of three 
second-magnitude stars, beginning on the 
east with Alpheratz and terminating near 
Perseus with Almaack. The central star is 
named Mirach. A few degrees north-west 
of Mirach glimmers the great Andromeda 
nebula. Below Andromeda, west of the 
meridian, appears the zodiacal constellation 
Aries, the Ram, indicated by a group of 
three stars, forming a triangle, the brightest 
of which is called Hamal. South-westerly 
from Aries is the zodiacal constellation Pisces, 
the Fishes, which consists mainly of faint 
stars arranged in pairs and running far toward 
the west along the course of the ecliptic, 
which crosses the equator at the vernal 
equinox, near the western end of the con- 
stellation. South of Pisces and Aries is 
the broad constellation Cetus, the Whale, 
marked by a number of large quadrilateral 
and pentagonal figures, formed by its stars. 
Near the centre of this constellation, but 
not ordinarily visible to the naked eye, is 
the celebrated variable Mira, also known as 
Omicron Ceti. 

With a little application any person can 



THe Constellations 257 

learn to recognise these constellations, even 
with the slight aid here offered, and if he 
does, he will find the knowledge thus acquired 
as delightful as it is useful. 



INDEX 



Aberration, 88 
Alpha Centauri, 145 
Alpha Draconis, 62 
Altitude, 13, 14 
Andromeda, nebula in, 236 
Antares, 24 
Aphelion, 8 

April, aspect of sky in, 251 
Arcturus, 224 
Asteroids, the, 187 
Atmosphere, the, 82 
August, aspect of sky in, 

253 
Azimuth, 13, 14 

Beta Aurigae, 230 

Binary (and multiple) stars, 

226 
Binaries, spectroscopic, 227 

Calendar, the, 117 

Calorie, measure of sun's 

heat, 129 
Cassiopeia, 22 
Cheops, pyramid of, 62 
Chromosphere, of sun, 135 
Circles, vertical, 15; alti- 
tude, 14; division of, 16 
Circumpolar stars, 224 
Clock, the astronomical, 

41,90 
Comets, 201, 203 
Constellations, the, 241 
Corona, of the sun, 135 



Coronium, 152 
Corona Borealis, 24 

Day, change of, 99, 101 
Day and night, 96 
December, aspect of sky in, 

256 
Declination, 31, 35 
Dipper, the Great, 21; the 

Little, 23 

Earth, description of the, 

67; weight of, 73 
Eclipses, 163 
Ecliptic, 43 
Elements, chemical, in the 

sun, 151 
Equator, 38 
Equinoxes, 45 
Eros, the asteroid, 142 

February, aspect of sky in, 
249 

Galaxy, the, 233 
Gravitation, laws of, 70 
"Greenwich of the Sky," 
32 

Heavens, apparent motion 

of, 19 
Helium, 152 

Horizon, 10, 12; dip of, 86 
Hour Circles, ^1 



259 



260 



Index 



January, aspect of sky in, 

248 
July, aspect of sky in, 252 
June, aspect of sky in, 252 
Jupiter, the planet, 189 

Kepler, laws of, 173 

Latitude, terrestrial, 30; 

celestial, 49, 51 
Light, pressure of, 206 
Longitude, terrestrial, 30; 

celestial, 49, 51 

Magnitudes, stellar, 220 
March, aspect of sky in, 250 
Mars, the planet, 181 
May, aspect of sky in, 251 
Mercury, the planet, 175 
Meteors, 210 
Meteorites, 214 
Milky Way, the, 233 
Molecules, escape from 
planetary atmospheres, 

*5 8 

Moon, description of, 153; 

how earth controls, 75 
Motion, proper, of solar 

system, 232 

Nadir, 1 1 

Nebulae, 236 

Nebular Hypothesis, 239 

Neptune, the planet, 199 

Noon-line, 12 

North Point, 20 

North Pole, 27, 37 

North Star, 20, 27, 61 

November, aspect of sky 

in, 255 
Nutation, 63 

Oblique sphere, 39 
October, aspect of sky in, 

254 
Oppositions, of Mars, 183 



Orbits of planets, 7, 52 
Orion, nebula in, 236 

Parallax, 136, 139 
Parallel sphere, 38 
Perihelion, 8 
Phases of the moon, 160 
Photography, in astronomy, 

240 
Photometry, of stars, 220 
Photosphere of sun, 134 
Planets, description of the, 

172 
Polar circles, 1 1 1 
Precession of the Equinoxes, 

55 

Refraction, 83, 85 
Right Ascension, 31, 35 
Right sphere, 39 
Rotation, effect of earth's, 



Saturn, description of, 195 
Seasons, the, 104, 107; 

secular change of , 1 1 5 
September, aspect of sky in, 

. 2 54 

Signs of Zodiac, 54 

Spectroscopic analysis, 145, 
147 ; shifting of lines, 227 

Sphere, celestial, 9 

Solstices, the, 47 

Stars, the fixed, 219; daily 
revolution of, 25; how 
to locate, 29; luminosity 
of, 222; spectra of, 221; 
variable, 225; temporary, 
235 ; double and multiple, 
226 

Sun, the description of , 127 

Sunspots, 131 

Temperature, of sun, 130 
Temporary stars, 235 
Tides, the, 76, 79 



Index 



261 



Time, 89; sidereal and solar, 

93 
Transits, of Venus, 140, 141 
Tropics, the, 1 1 1 

Universe, appearance of, 8 
Uranus, description of , 199 
Ursa Major, 21 



Variable stars, 225 
Vega, 63 

Venus, the planet, 178 
Vernal Equinox, 32, 42, 46 



Zenith, 10 
Zodiac, 50 



314.77-2X 



